Optics doi: 10.3390/opt7030040

Authors: Rajni Chaudhary Ashok Singh Bahota Neelam Agrawal Arti Yadav Ayush Shukla Veena Prasad Alejandro Pedro Ayala Swapnil Singh Poonam Tandon

Bent-core liquid crystals are renowned for their remarkable optical and ferro-electrical properties, making them highly sought after for various applications. However, to harness their full potential, a thorough understanding of their structural mechanisms and fluctuations during phase transitions is imperative. In this study, we conducted an in-depth analysis of the structural conformation of a V-shaped liquid crystal, specifically (E) 1,2-phenylene bis[4-((E)-(4-pentyloxy chloro phenyl) diazenyl) benzoate], referred to as V1, utilizing density functional theory (DFT) calculations at the B3LYP/6-311G(d,p) level. Geometry optimization and frequency calculations of the most stable conformers were performed at the same theoretical level. Our investigation into the mesomorphic behavior of V1 unveiled two enantiotropic phase transitions: Isotropic (Iso) → Nematic (N) → Smectic A (SmA) → Crystalline (Cry), with decreasing temperature. To elucidate the molecular alterations of V1 at the microscopic level, Fourier Transform Infrared (FT-IR) and Fourier Transform Raman (FT-Raman) spectra were recorded across various temperature ranges. Remarkably, the simulated vibrational spectra exhibited a striking resemblance to the experimentally observed vibrational spectra at room temperature, validating the accuracy of our computational approach. These findings hold immense promise for advancing further research and facilitating the development of novel applications leveraging the unique properties of bent-core liquid crystals.

Optics doi: 10.3390/opt7030039

Authors: Yuxuan Meng Xiaoyang Pan Mingzhong Pan Jin Yang Hongxing Qi

To circumvent the prohibitive cost of large-format infrared focal plane arrays and the significant spatial–spectral mismatch caused by spectral smile in conventional long-slit configurations, this work develops a low-cost short-wave infrared (SWIR, 1000–2500 nm) hyperspectral imaging system utilizing digital micromirror device (DMD) scanning paired with a single-element detector. A comprehensive analytical model for a prism–reflection grating (P-RG) compound dispersive element is established, enabling the joint optimization of the prism apex angle and grating period to achieve quantitative compensation of spectral distortion across the entire waveband. Based on this model, the optical system is integrated and optimized, while a centroid localization algorithm is implemented to facilitate online calibration of model parameters and real-time reconstruction of the hyperspectral data cube at the DMD plane. Experimental results demonstrate that both smile and keystone distortions are suppressed below 5μm throughout the 1000–2500 nm range, which is superior to the single DMD pixel pitch of 7.6μm. The full-field modulation transfer function (MTF) at the Nyquist frequency (32.9 lp/mm) exceeds 0.7, approaching the diffraction limit. Characterization confirms that the system provides 510 spectral channels with an average resolution of 3.57 nm and a spatial resolution of 2.5 μm. By effectively eliminating spectral overlap and cross-column crosstalk on the DMD encoding surface, this system provides a high-fidelity optical front-end for single-pixel imaging, offering a viable technical pathway for the development of affordable SWIR hyperspectral instrumentation.

Optics doi: 10.3390/opt7030038

Authors: Brandon R. Sulvarán-Salmoreno David Moreno-Hernández Diego Torres-Armenta

Accurate axial localization of microparticles is a key requirement in in-line digital holography (ILDH), particularly under noisy conditions and for weakly scattered objects. This work presents experimental and simulated benchmarking of three widely used focus metrics: maximum intensity, complex amplitude, and Kurtosis. Experimental holograms of microparticles with different diameters were recorded using a compact ILDH system, while simulated holograms of a 10 µm particle were generated. Numerical reconstruction was performed using a Fresnel convolution approach with FFT-based propagation over a range of axial distances. The performance of each focus metric was evaluated based on peak definition, robustness to coherent noise, and consistency across particle sizes and configurations. The results show that both maximum intensity and Kurtosis provide consistent and reliable axial localization, with very similar behavior across all cases. In contrast, the complex amplitude metric is more sensitive to noise and exhibits larger fluctuations in the axial response. These results indicate that simple intensity-based metrics can achieve accurate localization under moderate signal-to-noise conditions, while higher-order statistical metrics improve robustness in more challenging scenarios. This work provides practical guidelines for selecting autofocus criteria in ILDH systems for particle imaging and holographic metrology.

Optics doi: 10.3390/opt7030037

Authors: Junting He Xinyu Liu Donglin Fan Yuhang Xu Wenjuan Zhao Zhiwei Cui

In this work, we report a theoretical study of the energy, momentum, and angular momentum of non-diffracting Tricomi beams. By utilizing the vector potential in the Lorenz gauge, we derive the explicit analytical expressions for the electric and magnetic field components of non-diffracting Tricomi beams. A canonical theory is introduced to describe the energy, momentum, spin angular momentum (SAM), and orbital angular momentum (OAM) of the non-diffracting Tricomi beams. The effects of the asymmetry constants, topological charge, and half-cone angle on the energy, momentum, SAM, and OAM of the non-diffracting Tricomi beams are simulated and analyzed. This study provides fundamental physical insights into the dynamical characteristics of non-diffracting Tricomi beams relevant to potential optical manipulation applications.

Optics doi: 10.3390/opt7030036

Authors: Alessandro Bile Eugenio Fazio

We propose and analyze a new class of photonic neurons based on spatial solitons generated in photorefractive media. They are designed to operate entirely within the optical domain. By engineering single-node and multi-node multichannel architectures, we demonstrate the feasibility of constructing balanced, scalable, and reconfigurable structures capable of emulating neural behaviors such as symmetric signal splitting, plasticity, and dynamic adaptation. The optimization of geometric parameters—including soliton waveguides features, input distances, and incidence angles—proves crucial for ensuring the stability of solitonic propagation and the proper functioning of interaction nodes. The results lay the groundwork for the development of high-performance optical neural circuits, with potential applications in distributed signal processing, neuromorphic artificial intelligence, and reconfigurable optical memories.

Optics doi: 10.3390/opt7030035

Authors: Xikuan Wu Qian Zhang Hongying Ma Zhanwei Li Chenghai Pan Wenchang Zhang

This paper addresses the problem of counting stacked components in industrial scenarios and proposes a method that combines close-range scanning for complete contour acquisition with deep learning for quantity recognition: The contour acquisition system consists of a line array camera and a linear laser. Both are arranged horizontally at a certain angle, and the laser line is perpendicular and in the same direction as the stacking of the components. The system scans and connects single-row pixels along the stacking direction to obtain the contour. This method effectively avoids the occlusion problem caused by uneven stacking of components. The quantity recognition algorithm adopts a network structure similar to Encoding–Decoding using the component gap (cls: 0 indicates not, 1 indicates yes) and the endpoint coordinates of the separation line segment [cls, x1, y1, x2, y2] to form a label. Multi-scale anchors are introduced to predict the translation distance of the line segment (positive or negative, indicating direction). The prediction head is fully convolutional, and the loss for regression is computed using the predicted endpoints of the ground-truth line segments. A line segment redundancy removal method is proposed to output the predicted confidence (conf) and coordinates [conf, px1, py1, px2, py2] for each component gap. The self-built dataset is used for training and validation. Experiments show that the recognition accuracy of each image reaches 95.79%, and the gap recognition accuracy reaches 99.62%, which can meet the requirements of automation.

Optics doi: 10.3390/opt7030034

Authors: Grazia Giuseppina Politano

The teaching of chaos and nonlinear dynamics remains a significant challenge in physics education, as these concepts are often introduced through abstract mathematical models that are difficult to visualize. In this work, we propose an experimental approach based on electro-optics in nematic liquid crystals as an effective and accessible platform for teaching these phenomena. In particular, the system exhibits a transition from ordered convective patterns to strongly disordered turbulent regimes, which can be directly observed in real time using simple optical techniques. This experimental framework enables students to explore key concepts of nonlinear physics, including instability thresholds, pattern formation, and the emergence of complex dynamical behavior. The transition occurs through the nucleation and growth of turbulent domains, facilitating the understanding of nonequilibrium dynamics. From a pedagogical perspective, the proposed experiment combines strong visual impact with experimental controllability and accessibility, making it suitable for undergraduate students in physics, mathematics, and engineering. Furthermore, the integration of AI-assisted analysis provides students with an accessible framework to process experimental data, identify dynamical regimes, and explore complex systems through novel data-driven methodologies.

Optics doi: 10.3390/opt7030033

Authors: Boris Dobrev Aneliya Dakova-Mollova Valeri Slavchev Diana Dakova Zara Kasapeteva Lubomir Kovachev

The present work investigates the linear regime of propagation of modulated vector optical fields in isotropic dispersive media by focusing on the formation of complex vector vortex structures with amplitude-type singularities. A mathematical algorithm designed to derive novel exact analytical solutions for the linear vector amplitude equation is presented, enabling the systematic development and classification of diffraction-free vector solutions. Various types of solutions for the two orthogonal components of the vector amplitude function are obtained, resulting in non-trivial spatial amplitude structures in their cross sections. The proposed approach allows for precise analytical governance of the spatial and polarization properties of the obtained vortices via the vortex parameter n. The presented model offers a comprehensive framework for generating different types of vector vortex structures by choosing the values of the parameters n and m, depending on the initial phase of the components. The derived solutions extend the capabilities of conventional phase modulation techniques. It is demonstrated that by changing the vortex parameter n, the structural complexity of both the amplitude distributions and polarization patterns increases. A number of numerical simulations, based on the obtained analytical solutions, are performed. They validate the model and clearly illustrate the characteristic vectorial features via detailed vector diagrams.

Optics doi: 10.3390/opt7030032

Authors: Huiping Shen Yuhao Kang Guojian Jiang

The development of efficient, single-phase-excitable white-light phosphors remains a critical challenge for solid-state lighting applications. In this work, white-light-emitting CaWO4:Eu3+/g-C3N4 composites were successfully developed by integrating red-emitting CaWO4:7%Eu3+ with blue-emitting graphitic carbon nitride (g-C3N4). Under 365 nm near-UV excitation, the composite exhibits dual-band emission originating from the 5D0 → 7F2 transition of Eu3+ (~616 nm) and the intrinsic band-edge luminescence of g-C3N4 (~460 nm). The optimal white-light performance is achieved at a g-C3N4 content of 0.5 wt%, yielding CIE chromaticity coordinates of (0.294, 0.324) and a correlated color temperature (CCT) of 7673 K. This sample demonstrates a photoluminescence quantum yield (PLQY) of 3.25%. Moreover, the CaWO4:Eu3+/g-C3N4 composite shows enhanced thermal stability, retaining 78% of its initial emission intensity at 175 °C, with an activation energy of 0.41 eV—significantly higher than that of the pristine CaWO4:Eu3+ (0.22 eV). These results indicate that the CaWO4:Eu3+/g-C3N4 heterostructured phosphor is a promising candidate for single-phase-excitable white-light applications.

Optics doi: 10.3390/opt7030031

Authors: Amr G. AbdElKader Ahmed Allam Hossam M. Shalaby Kazutoshi Kato

Reconfigurable intelligent surfaces (RISs) have recently attracted attention as a potential solution for improving the reliability of optical wireless communication links, especially when direct transmission (DT) becomes severely degraded due to dynamic channel conditions. In this study, an RIS-assisted architecture based on a modulating retroreflector is proposed for underwater optical wireless communications (MRR-UOWC). In the considered system, both the DT path and the RIS-assisted path transmit the same information simultaneously at the same data rate. The propagation channels are modeled by taking into account propagation loss, Gamma–Gamma turbulence, and pointing error effects. At the receiver, the signals arriving through the direct path and the RIS-reflected path are coherently combined. To evaluate the effectiveness of this configuration, two diversity combining techniques, namely selection combining (SC) and maximum ratio combining (MRC), are investigated. Closed-form analytical expressions for the outage probability (Pout), average bit-error rate (BER), and ergodic capacity (C¯) are derived using the probability density function (PDF), cumulative distribution function (CDF), and moment-generating function (MGF) of the end-to-end signal-to-noise ratio (SNR). The analysis indicates that jointly exploiting the DT and RIS-assisted links can provide noticeable performance gains by leveraging the complementary characteristics of the two propagation paths.

Optics doi: 10.3390/opt7030030

Authors: Jingwen Wang Shuang Zhang Wei Cheng Zhixue Xing Shengying Fan Galina Melnikova Vasilina Lapitskaya Shoufa Di Jincheng Ni

The precise fabrication and controllable actuation of magnetic microspheres hold significant application value in biomedicine, microfluidic chips and other fields. Based on femtosecond laser two-photon polymerization technology (FLTPP), two methods are adopted to prepare magnetic microspheres in this study. Magnetic microspheres are fabricated via photoresist modification and post-treatment processes. Meanwhile, a 3D magnetic actuation system composed of a three-axis movable magnetic drive module and a real-time imaging system is constructed, enabling the flexible 3D actuation and real-time dynamic monitoring and visualized observation of magnetic microspheres. The results demonstrate that the magnetic microspheres exhibit sensitive magnetic response characteristics. The constructed magnetic actuation system features large travel range (XY: ±6.5 mm, Z: 10 mm), high precision (20 μm) and flexible manipulation, enabling stable locomotion of the microrobots in straight channels, L-shaped channels, and square channels. This study provides a technical reference for the fabrication and manipulation of magnetic micro/nano devices, and lays a foundation for their subsequent integrated applications in microfluidic systems.

Optics doi: 10.3390/opt7020029

Authors: Xingyi Wang Jingge Wang Qiqi Liu Yumeng Li Xiaoyan Lin

Waste electronic components are valuable secondary resources containing various metals. Analyzing their elemental distribution is crucial for developing recycling methods. Micro- X-ray fluorescence (μ-XRF) is commonly used for this purpose, but traditional polychromatic X-ray excitation creates high background scattering. This masks trace element signals, impairing detection limits and accurate identification of minor valuable or hazardous elements. To address this, this study developed a monochromatic μ-XRF spectrometer using a low-power molybdenum-target X-ray tube. The system integrates polycapillary lenses for X-ray regulation and a flat crystal for monochromatization, producing a micron-sized monochromatic X-ray spot with high power density. This design eliminates scattered background from the primary continuous spectrum and enhances excitation efficiency by concentrating photon flux, enabling high-brightness monochromatic beams even at low tube power. The spectrometer was validated by analyzing a waste printed circuit board. High-resolution elemental mapping successfully revealed clear distribution patterns of major elements like copper, nickel, and iron, consistent with their physical structures. These images allowed intuitive differentiation of compositional differences across functional regions. This technique effectively overcomes the background interference caused by polychromatic excitation and is expected to further enhance the quality and reliability of elemental distribution imaging. It provides a powerful tool for formulating precise, scientific recycling strategies for waste electronics.

Optics doi: 10.3390/opt7020028

Authors: Antonio Currà Riccardo Gasbarrone Andrea Maffucci Giuseppe Capobianco Giuseppe Bonifazi Andrea Cervia Carlo Trompetto Paolo Missori Silvia Serranti

Near-infrared spectroscopy (NIRS) is increasingly studied as a non-invasive optical investigation tool for in vivo tissue characterization, including applications to skeletal muscle and brain regions. In this context, previous studies have demonstrated reliability in differentiating muscle sites, typically relying on dense acquisition schemes (≥50 spectra acquired per site) to ensure signal stability. However, this requirement may limit throughput and hinder real-world clinical translation. Optimizing the trade-off between acquisition burden and classification performance represents a key design problem for device scalability and feasibility of bedside deployment. In this study, we explored the impact of spectral sampling density on machine learning-based muscle discrimination. Thirty healthy adults provided 50 Vis–SWIR (Visible–Short-Wave Infrared; 350–2500 nm) reflectance spectra per biceps and triceps muscle sites (3000 spectra). Seven datasets were generated by random subsampling, progressively reducing the number of spectra (from 50 to 1 spectra/muscle/subject). All datasets underwent an identical preprocessing pipeline and were subjected to Partial Least-Squares Discriminant Analysis (PLS-DA) classification. PLS-DA achieved near-perfect discrimination from 50 to 5 spectra per muscle with a mean cross-validation (CV) accuracy ≥ 99.5%, whereas performance collapsed abruptly at three spectra (CV accuracy ~39%) and one spectrum (CV accuracy ~15%). Therefore, high machine learning classification performance is retained even when the number of acquired spectra is substantially reduced. These findings support the feasibility of acquisition-efficient protocols that may enhance device portability and reduce measurement time, thus enabling NIRS integration into clinical workflows. From a biomedical engineering standpoint, spectra number reduction without loss of predictive performance represents a key step toward scalable, real-time, and patient-centered Vis–SWIR diagnostic platforms.

Optics doi: 10.3390/opt7020027

Authors: Zara Kasapeteva Anelia Dakova-Mollova Diana Dakova Kamen Kovachev Lubomir Kovachev Anjan Biswas

In the present work a new regime of periodical energy exchange between pump, signal and idler waves, under the influence of the process of four-wave mixing (FWM), with additional consideration of the effects of self-phase modulation (SPM) and cross-phase modulation (XPM), is presented. In our previous papers a theoretical model which successfully describes the amplification and periodic energy exchange between the three optical waves in CW regime of laser source propagation (short-cut equations) was developed. Exact analytical solutions, describing the periodic changes in the intensities of pump, signal and idler waves, were found and expressed by the Jacobi elliptic functions. The period of the energy exchange between the waves can be presented by elliptic integral of the first kind. In the current research, the periods of energy exchange between the pump, signal and idler waves in the process of FWM, additionally taking into account the effects of SPM and XPM, are investigated. A comparison between the obtained results has been made. It is shown that the effects of self-phase modulation and cross-phase modulation increase the period of energy exchange.

Optics doi: 10.3390/opt7020026

Authors: Wenrong Mo Bin Li Jianxin Wang Cai Wen Youlin Wang Awais Tabassum

Lasers are a key enabling technology across numerous engineering and scientific fields, especially in high-energy laser systems for defense, materials processing, and fusion research, where precise characterization of high-power, large-divergence-angle laser spots is critical. However, the inherent properties of high-power, large-divergence-angle lasers—such as large spot area and strong intensity contrast—pose real obstacles to existing methods, which often suffer from low accuracy and inefficiency. In this paper, a flat-field correction technique was proposed for the CCD to reduce the distortions produced by the non-uniform response of the sensor in spot measurements. Then, a spot recognition algorithm based on support vector machines was developed, which can effectively and accurately locate and identify laser spots with limited training samples and computational resources, achieving a classification accuracy of over 98.11%. Additionally, an efficient correction approach is proposed to assess the spot intensity and shape with high accuracy even at large tilt angles. Experimental results show that this proposed approach can measure the high-power laser spot with a large divergence angle precisely and efficiently, and improves both the measurement precision and operational efficiency remarkably.

Optics doi: 10.3390/opt7020025

Authors: Kamil Zuber Duncan Wright Jebum Choi Joni Sytsma Colin Hall

A compact, low-cost star tracker system tailored for small satellite applications was designed and prototyped. The system was designed with a fast f/1.2 aperture, a 20 × 13° field of view, and a theoretical angular resolution of 10 arcs—sufficient for the determination of attitude and orbit of a satellite. The optical design is based on a monolithic Maksutov–Cassegrain architecture, with lens assemblies fabricated from CR39 or PMMA to eliminate collimation requirements and improve vibration resistance. The lens was machined using Single-Point Diamond Turning to a precision better than λ/14. It was coated with a multilayer antireflective and highly reflective coatings applied via magnetron sputtering to reduce stray reflections and improve light throughput. The housing was produced using electron beam powder-bed fusion with Ti-64 alloy, while the use of commercial imaging sensors minimizes overall cost. Prototype testing confirmed to plate-solve star patterns with precision better than 27 arcs at 100 ms imaging time across all analysed images.

Optics doi: 10.3390/opt7020024

Authors: Linbo He Jing Cao Wenhai Gao Yang Liao Yan Xue Cong Chen Ke Liu Xupeng Yuan Jijun Feng Huiyu Chen Yuxin Leng

Diamond antireflection techniques are of high interest for optical windows operating at extreme conditions. Herein, diamond antireflective microstructures in mid-infrared (MIR) spectral range were theoretically designed and experimentally fabricated. Finite difference time domain (FDTD) simulations were used to optimize the transmission performance of the diamond microstructures. Based on the simulation results, the optimized microstructures were fabricated by femtosecond (fs) laser direct writing (1030 nm, 300 fs, 25 kHz) followed by wet etching. After wet etching, the laser-modified zones and the accumulated graphitized clusters were effectively removed, thereby achieving the desired depth. The influences of laser power and scanning strategy on the morphology evolution of diamond microstructures were investigated. It was found that at the optimal conditions, the transmittance of the diamond increased from 70.9% to 81.4% (single-side) over a broad spectrum from 8 to 22 μm. This work demonstrates a promising hybrid fs laser/wet etching technique for diamond antireflective microstructures in MIR spectral range.

Optics doi: 10.3390/opt7020023

Authors: Abigail Deaton Hengzhou Liu Nathan J. Dawson

Polydimethylsiloxane (PDMS) films doped with graphene nanoplatelets (GNP) with an embossed surface-relief grating were investigated as photothermal actuated sensors. The films were initially characterized using controlled environmental heating where the wavelength of a diffracted white-light probe beam measured at a fixed angle increased monotonically with temperature due to thermal expansion of the grating. An asymmetric double sigmoidal function tracked the shift in peak diffraction wavelength. The observed thermal response is consistent with the thermal expansion of a freestanding PDMS composite film. When a continuous-wave (CW) laser was incident on the film, intensity-dependent photothermal expansion caused a transient deformation in the grating. The photomechanical behavior of the grating, tracked by the diffracted probe beam with a miniature spectrometer, was then shown to act as a laser power meter. These results demonstrate that photomechanical materials can be used as add-ons to existing optical spectroscopy devices for power-sensing applications.

Optics doi: 10.3390/opt7020022

Authors: Karan K. Singh Guillermo E. Sánchez-Guerrero Perla M. Viera-González Carlos A. Fuentes-Hernandez María T. Romero de la Cruz Eduardo Martínez-Guerra Rodolfo Cortés-Martínez Edgar Martínez-Guerra

Surface plasmon resonance (SPR) sensors based on nanostructured metasurfaces offer enhanced sensitivity through engineered electromagnetic responses. In this study, we present an analytical and numerical investigation of the plasmonic behavior of gold nanopillar (Au-NP) and nanohole (Au-NH) arrays under both p- and s-polarized illumination, employing the Effective Medium Theory (EMT) in combination with the Transfer Matrix Method (TMM). The study combines Effective Medium Theory (EMT) and the Transfer Matrix Method (TMM) to describe the macroscopic optical response of multilayer plasmonic systems. For p-polarization, the nanostructure geometry strongly modulates the real and imaginary parts of the effective permittivity, with nanoholes supporting stronger SPR coupling and reduced optical losses compared to nanopillars. Under s-polarization, the effective permittivity remains largely invariant, primarily driven by the filling fraction. The analysis reveals that polarization-dependent behavior arises from boundary-condition-mediated coupling mechanisms governing surface plasmon excitation, aligning with classical plasmonic theory. Benchmarking against analytical dispersion relations and published experimental data for Au/BK7 systems shows close agreement within ±2°, confirming the physical consistency of the EMT–TMM framework. These results provide a systematic description of how polarization and filling fraction jointly modulate SPR coupling. The results offer a foundation for the rational design of plasmonic coatings and SPR-supporting metasurfaces by elucidating macroscopic coupling trends; however, no quantitative sensor performance metrics, such as refractive index sensitivity or figure of merit, are evaluated in this work.

Optics doi: 10.3390/opt7020021

Authors: Soufyane Aqiqi Elarbi Laghchim C. A. Duque

In this work, a comprehensive first-principles investigation of the electronic, magnetic, and optical properties of pristine and Fe-, Co-, and Ni-doped MoS2 monolayers is presented within the framework of density functional theory. Substitutional transition-metal doping at the Mo site is shown to induce spin-polarized impurity states within the pristine band gap, leading to significant modifications of the electronic structure, including metallic, semimetallic, or half-metallic behavior depending on the dopant species. The calculated spin-resolved band structures and projected density of states reveal a strong hybridization between the dopant 3d orbitals and the Mo-4d/S-3p states, giving rise to sizable magnetic moments and dopant-dependent exchange splitting. When spin–orbit coupling is included, the combined effect of exchange interactions and relativistic effects leads to an effective valley splitting at the K and K′ points, whose magnitude and sign depend sensitively on the chemical nature of the dopant. Optical properties are analyzed within a linear-response framework, showing pronounced dopant-induced modifications of the optical spectra. While the pristine monolayer exhibits well-defined excitonic features, transition-metal substitution introduces low-energy optical transitions associated with impurity-related states. Consequently, the exciton binding energies estimated from the difference between the electronic and optical gaps are interpreted as effective measures of dopant-induced perturbations to optical transitions, rather than as quantitative many-body excitonic binding energies in the strict sense. These results provide microscopic insight into the interplay between magnetism, spin–orbit coupling, and optical response in doped MoS2 monolayers, highlighting the potential of transition-metal substitution as a route to engineer spin- and valley-dependent phenomena in two-dimensional materials.

Optics doi: 10.3390/opt7020020

Authors: Jia Liu Ruikai Zhang Yangdong Zhou Dewu Li Yixin Wang Lu Yin

The echelle spectrometer utilizes an echelle grating as the primary dispersive element, combined with a prism or planar grating for cross-dispersion, to form a two-dimensional spectral image on an area-array Charge-Coupled Device (CCD). Compared with traditional spectrometers, this configuration provides superior spectral resolution, broader wavelength coverage, enhanced transient direct-reading capability, and higher energy throughput within a similar footprint. However, the use of area-array detectors significantly increases system cost, limiting adoption in cost-sensitive applications. To reduce cost while maintaining performance, we introduce a digital micromirror device (DMD) as a spatial light modulator to replace the traditional area-array detector, paired with a highly sensitive photomultiplier tube (PMT) for signal acquisition. The designed system operates across a wavelength range of 270 to 800 nm within a compact footprint of approximately 307 mm × 210 mm × 150 mm. The focused spot is accurately positioned on the DMD surface across the entire band, with the root mean square (RMS) spot radius smaller than a single micromirror’s size. Spectral information is efficiently coupled into the PMT via a focusing mirror by selectively flipping the DMD micromirrors for detection.

Optics doi: 10.3390/opt7020019

Authors: Stefani Petrova Kalina Ivanova Iliana Ivanova Albena Bachvarova-Nedelcheva

In this study, sol–gel-synthesized nanoparticles were characterized by various physicochemical techniques, including scanning electron microscopy (SEM), X-ray powder diffraction (XRD), UV-Vis spectrophotometry, and thermogravimetric analysis (DTA/TG). The as-obtained powders were tested for their antimicrobial activity against the Gram-positive bacteria Staphylococcus aureus and Enterococcus faecalis, as well as the fungal strains Candida albicans and Saccharomyces cerevisiae. Additionally, the photocatalytic performance of the samples was evaluated under simulated solar light. The results are promising for possible environmental applications. The antimicrobial assessment also revealed notable effects, with varying degrees of growth inhibition observed across the tested microorganisms. The main approach in this study consists of the combination of physicochemical characterization with antibacterial and photocatalytic evaluations, resulting in promising multifunctional materials.

Optics doi: 10.3390/opt7020018

Authors: Jian Qiao Junxi Zhu Yuemei Huang Xiaoqi Cheng Jingwei Yang Guojie Lu Haishu Tan

Optical inspection of highly reflective cylindrical components—such as stainless-steel vessels featuring both planar and curvilinear surfaces—presents significant challenges due to complex geometric distortions in single-pass imaging. This study proposes a line-scan imaging framework that integrates synchronized kinematic control with geometry-aware distortion correction. The system addresses shape deformations through three coordinated modules: (1) parametric synchronization between rotational motion and image acquisition ensures full-surface coverage; (2) scanline-specific 1D projective transformations correct perspective distortions on toroidal sidewalls; and (3) adaptive polar coordinate remapping restores radial symmetry on circular bases. Experimental results demonstrate subpixel-level geometric correction accuracy, validating the proposed framework’s effectiveness in eliminating geometric aberrations with low computational complexity and without reliance on data-driven training, while maintaining compatibility with defect detection and quantitative surface analysis of specular cylindrical specimens.

Optics doi: 10.3390/opt7010017

Authors: Bing Zhou Chao Wang Xiaona Li Liang Dong Ran Wang Rui Li

Wool fibers undergo significant structural changes during industrial stretching, which directly impact their mechanical properties and textile performance, making monitoring of the stretching process essential for optimizing wool products. In this study, we demonstrate the effective use of polarized second harmonic generation (P-SHG) imaging for monitoring the wool fiber stretching process. P-SHG is highly sensitive to non-centrosymmetric structures, enabling clear observation of changes in α-keratin alignment and the reconstruction of cortical interfaces during stretching. Quantitative P-SHG analysis revealed a significant decrease in the effective pitch angle (θe) from 54° ± 1° to 33° ± 3° after stretching, confirming the dipole orientation changes in keratin molecules. These findings were further validated through additional characterization techniques, including scanning electron microscopy (SEM), polarizing optical microscopy (POM), X-ray diffraction (XRD), and Raman spectroscopy (RS). The results show that the industrial stretching process of wool alters the morphology at the surface scale, enhances the alignment of macroscopic fibers, and induces a transition from α-helix to β-sheet. Our technique is simple, effective, and capable of in situ monitoring of the structural changes in wool fibers, making it highly promising for applications in the wool industry.

Optics doi: 10.3390/opt7010016

Authors: Henk F. Arnoldus

We have studied the extinction power flow for a dipole in a laser beam, and embedded in a dissipating medium. The power flows along the field lines of the Poynting vector. We have shown that near the particle, the field lines form closed loops, which start and end at the location of the dipole. A closed-form expression for these loops has been derived, and we have shown how the orientation direction of a loop is determined by the permittivities and permeabilities of the host medium and the particle. It is also shown that the spatial extent of these loops is determined by singularities in the flow pattern. It is shown that the extent of the loop structure near the dipole diminishes strongly when there is dissipation in the medium. This is due to the appearance of singularities very close to the particle, which are due to the damping. At greater distances, flow lines run off to the far field or they come in from the far field. Most flow lines change from incoming to outgoing, or vice versa, so they turn around somewhere in the flow field. Singularities, points where the Poynting vector vanishes, appear on the coordinate axes. At these points, field lines split. Off the axes, singularities appear as the centers of vortices. Near a vortex, energy swirls around the singular point. Field lines can come out of the center of a vortex or end there.

Optics doi: 10.3390/opt7010015

Authors: Huiming Xiao Jiahui Wu Haoyang Lin Lihao Wang Jianfeng He Leqing Lin Ruobin Zhuang Guantian Hong Jiabao Xie Jianhui Yu Wenguo Zhu Yongchun Zhong Zhigang Song Huadan Zheng

We report a deep learning-assisted quartz-enhanced photoacoustic spectroscopy (QEPAS) sensor for trace water vapor detection in air. A 1392 nm butterfly-packaged DFB laser is wavelength-modulated at f0/2, and the QEPAS signal is retrieved by second-harmonic (2f) lock-in demodulation using a commercial quartz tuning fork gas cell. After optimizing the modulation depth to 400 mV, a 1D U-Net denoising network trained with pseudo-clean supervision is applied to the measured 2f traces, yielding an SNR improvement of 2.05× (3.11 dB). Allan deviation analysis indicates a minimum detection limit (MDL) of ~2.21 ppm at an optimum averaging time of ~619 s, corresponding to an ~2.1× improvement compared with the raw output. These results demonstrate that neural-network-based post-processing can improve QEPAS water vapor sensing performance without modifying the optical hardware.

Optics doi: 10.3390/opt7010014

Authors: Juan de Dios Unión-Sánchez Manuel Jesus Hermoso-Orzaez Carmen Borrás-Rodríguez Julio Terrados-Cepeda

Glare is a critical factor in the design of LED luminaires for street lighting, particularly in environments where pedestrians, cyclists and drivers coexist. Generally, glare assessments are performed for fixed geometries and a single observer, limiting their applicability to real urban environments. This study examines the effect of angular redistribution of the beam on glare and illuminance by introducing the relative angular parameter α into the photometric model and the UGR calculation. A generic LED luminaire is modelled using a cosine-type luminous intensity distribution raised to a power, and the emitting surface is also discretized to evaluate the luminance, solid angle and Guth position index at the patch level. This approach is applied to three distinct observer geometries—pedestrian, cyclist and driver—allowing direct comparison using a unified mathematical formulation. The results show that beam redistribution affects each observer differently, reducing glare for pedestrians while simultaneously increasing it for drivers, whereas cyclists show limited sensitivity to angular changes. Although relative illuminance and UGR show similar monotonic trends, their physical and perceptual interpretation is different. This paper presents a novel tool for the preliminary analysis of trade-offs between visual comfort and luminous efficiency in urban lighting design.

Optics doi: 10.3390/opt7010013

Authors: Luca Guarino Giovanni Cannarozzo Luca Gargano Elena Zappia Alessandro Clementi Mario Sannino Giovanni Pellacani Steven Paul Nisticò

Background: Keloids are fibroproliferative scars with a prominent vascular component, and pulsed dye laser (PDL) is an established treatment, but objective imaging biomarkers of response are lacking. Objective: To evaluate whether dynamic optical coherence tomography (D-OCT) can provide quantitative, depth-resolved monitoring of keloid vascular remodeling under PDL and to explore candidate metrics for hypothesis-generating assessment in future studies. Methods: We conducted a prospective single-case pilot, hypothesis-generating study of a thoracic keloid treated with three sessions of 595 nm PDL, acquiring D-OCT scans at baseline and approximately 30, 60, and 90 days over a standardized 4 × 4 mm region of interest at 0.15, 0.30, and 0.50 mm depths. Primary D-OCT metrics included vascular en-face area, vessel length density, junction density, and mean vessel caliber. Results: The superficial layer (0.15 mm) showed an almost complete collapse of vascular signal (area −88% vs. baseline), the intermediate layer at 0.30 mm exhibited a sustained ~39% reduction in vascular area with parallel decreases in length and caliber at stable branching, and the deep layer at 0.50 mm showed modest area changes with longer but thinner vessels. These depth-resolved changes were consistent with clinical improvement in Vancouver Scar Scale and POSAS scores. Conclusions: D-OCT yielded quantitative, clinically interpretable vascular metrics that align with the expected effects of PDL in this single patient. In this patient, the percentage reduction in vascular area at 0.30 mm by week 8 emerged as a candidate quantitative metric for response monitoring; thresholds in the order of ≥25% could be tested prospectively as hypothesis-generating cut-offs in future controlled and reliability-tested studies, but are not proposed here as validated clinical criteria.

Optics doi: 10.3390/opt7010012

Authors: Cheng Li Bing Wu Yu Zhang Hang Zhu Zhigang Gao Jie Zhang Linghao Kong Xiaojun Cui Guoyu Zhang Feng Peng

A composite fiber optic sensor based on a misaligned peanut-shaped structure and the single-mode fiber–multimode fiber–single-mode fiber (SMS) structure is proposed for simultaneous strain and temperature measurements. The misaligned peanut-shaped structure is formed by introducing a certain core-offset during fusion splicing. Through a simulation analysis of the sensor, the optical field distribution of the sensor structure under different offset amounts is obtained. The experimental results demonstrate that the sensor achieves a maximum strain sensitivity of −48.21 pm/µε with an offset of 35.61 µm under a strain range of 0–600 µε and a maximum temperature sensitivity of 124.29 pm/°C at a 24.35 µm offset with a temperature range of 35–95 °C. Meanwhile, the sensor with a 35.61 µm offset has two resonance peaks that are selected for simultaneous measurements, with strain sensitivities of −48.21 pm/µε and −47.04 pm/µε and temperature sensitivities of 75.71 pm/°C and 84.29 pm/°C, respectively. Therefore, the simultaneous measurement of the strain and temperature can be achieved through a matrix method, demonstrating that the sensor possesses a dual-parameter sensing capability for the strain and temperature.

Optics doi: 10.3390/opt7010011

Authors: Bochao Zhang Minyan Zhang Lei Li Jianglang Bie Shuoyi Jiao Zhuanzhuan Guo Xinjie Cai Bowen Hou

In this study, a double-lattice photonic crystal structure was designed to achieve deep ultraviolet lasing without the use of any Distributed Bragg Reflector (DBR), which is called a photonic-crystal surface-emitting laser (PCSEL). The plane wave expansion (PWE) method was used to study the influence of various structural parameters on the resonant wavelength. Utilizing the random forest algorithm, we determined that the importance of the lattice constant to the resonant wavelength is 95.24%. Furthermore, we realized the reverse design of double-lattice photonic crystals from the target wavelength to optimal structural parameters through a radial basis function (RBF) network algorithm. Comparative analysis of the extreme learning machine (ELM) and back propagation (BP) algorithms demonstrated that RBF-based performance was notably superior to the training outcomes of other algorithms. The mean absolute error (MAE) of the lattice constant of the test set in the training results was 0.7610 nm, root mean square error (RMSE) was 1.143×10-3 nm, and mean absolute relative error (MARE) was 5.489×10-3. We verified the reliability of the algorithm and designed 13 groups of photonic crystals with different epitaxial structures. The mean square error (MSE) was 0.6188 nm2 compared with that of the plane wave expansion method. This work demonstrates applicability across various wavebands and epitaxial structures in GaN-based devices, providing a novel approach for the rapid iteration of deep ultraviolet PCSELs.

Optics doi: 10.3390/opt7010010

Authors: Cinzia Casu Antonia Sinesi Andrea Butera Sara Fais Alessandro Chiesa Andrea Scribante Germano Orrù

The growing prevalence of multidrug-resistant (MDR) Candida spp. necessitates the development of new antifungal strategies. Photodynamic therapy (PDT), already widely used in the treatment of various oral infections, is based on the synergistic interaction of three key elements: a photosensitizer capable of selectively binding to microbial cells, a light source with the appropriate wavelength, and the presence of molecular oxygen. This interaction results in the production of singlet oxygen and reactive oxygen species, responsible for the selective destruction of microorganisms. In recent years, numerous natural compounds have been explored as potential photosensitizers. Olive oil, a cornerstone of the Mediterranean diet, was recently recognized by the U.S. Food and Drug Administration as a medicinal substance thanks to its soothing, immunomodulatory, and antimicrobial properties, which have also been documented in regard to oral administration. Materials and Methods: The aim of this in vitro study was to evaluate the efficacy of activated olive oil as a novel photosensitizer in PDT against Candida species. Oral MDR clinical isolates of C. albicans, C. krusei, and C. glabrata were analyzed using the Kirby–Bauer method according to EUCAST protocols. Six different experimental conditions were considered for each strain: (i) 100 μL of extra-virgin olive oil (EVOO); (ii) 100 μL of EVOO pre-activated with 3% H2O2 (EVOO-H); (iii) 100 μL of EVOO irradiated for 5 min with polarized light (480–3400 nm, 25 W); (iv) 100 μL of EVOO-H subjected to the same polarized light; (v) 100 μL of EVOO irradiated for 5 min with a 660 nm diode laser (100 mW); and (vi) 100 μL of EVOO-H irradiated with the same laser. All plates were incubated at 37 °C for 48 h. Results: The results showed a variable response among the different Candida species. C. glabrata showed sensitivity to all experimental conditions, with a 50% increase in the diameter of the inhibition zone in the presence of polarized light. C. krusei showed no sensitivity under any of the conditions tested. C. albicans showed antifungal activity exclusively when EVOO-H was activated by light. In particular, activation of EVOO and EVOO-H with polarized light resulted in the largest inhibition zones. Conclusions: In conclusion, olive oil, both alone and pre-activated with hydrogen peroxide, can be considered an effective photosensitizer against drug-resistant Candida spp., especially when combined with polarized light.

Optics doi: 10.3390/opt7010009

Authors: Jinhyung Lee Geunwoong Jeon Byeongjun Yoon Donghyun Kim Hyeunwoo Kim Sun-Myong Kim

We investigate the structure of correlation singularities for the Laguerre–Gauss beam of order n=0 and m=2 in the transverse plane during the propagation of the beam in the beam-wander model. We explicitly derive analytical expressions for the cross-spectral density of the corresponding beam order and the analytic expressions representing the singular behavior. We also verify that the singular points disappear at certain z values and reappear at other z values as shown in the previous numerical study. We investigate the dependence of the absolute value of the complex degree of coherence μ on the parameter δ of the beam-wander model during the propagation of the Laguerre–Gauss beam in the corresponding order. The complex degree of coherence depends not only on δ but also on the relative positions of two transverse observation points ρ1 and ρ2, as well as on the propagation variable z for the fixed values of the beam waist and the wavelength of the Laguerre–Gauss beam. Experiments on μ can demonstrate the range of the applicability of the beam-wander model in the turbulent atmosphere. Finally, we examine the orbital angular momentum flux density of the beam and confirm that the general behaviors of the previous studies also hold for m=2.

Optics doi: 10.3390/opt7010008

Authors: Vasyl G. Kravets Vasyl Petruk Serhii Kvaterniuk Roman Petruk

Organic optoelectronic devices receive appreciable attention due to their low cost, ecology, mechanical flexibility, band-gap engineering, brightness, and solution process ability over a broad area. In this study, we designed and studied organic light-emitting diodes (OLEDs) consisting of an assembly of natural dyes, extracted from noble fir leaves (evergreen) and blue hydrangea flowers mixed with poly-methyl methacrylate (PMMA) as light emitters. We experimentally demonstrate the effective conversion of blue light emitted by an inorganic laser/photodiode into longer-wavelength red and green tunable photoluminescence due to the excitation of natural dye–PMMA nanostructures. UV-visible absorption and photoluminescence spectroscopy, ellipsometry, and Fourier transform infrared methods, together with optical microscopy, were performed for confirming and characterizing the properties of light-emitting diodes based on natural dyes. We highlighted the optical and physical properties of two different natural dyes and demonstrated how such characteristics can be exploited to make efficient LED devices. A strong pure red emission with a narrow full-width at half maximum (FWHM) of 23 nm in the noble fir dye–PMMA layer and a green emission with a FWHM of 45 nm in blue hydrangea dye–PMMA layer were observed. It was revealed that adding monolayer MoS2 to the nanostructures can significantly enhance the photoluminescence of the natural dye due to a strong correlation between the emission bands of the inorganic–organic emitters and back mirror reflection of the excitation blue light from the monolayer. Based on the investigation of two natural dyes, we demonstrated viable pathways for scalable manufacturing of efficient hybrid OLEDs consisting of assembly of natural-dye polymers through low-cost, purely ecological, and convenient processes.

Optics doi: 10.3390/opt7010007

Authors: Agnieszka Jablonska Bong Lee R. Max Petty Danh Pham Rajveer Sagoo Trang Thien Pham Zygmunt Gryczynski Ignacy Gryczynski

We report the first observation of room-temperature phosphorescence (RTP) of quinine sulfate (QS) in poly (vinyl alcohol) (PVA) films. Steady-state and time-gated measurements were performed to characterize the phosphorescence spectra, anisotropies, and lifetimes to estimate the phosphorescence properties. The RTP response of organic emitters in polymer matrices is particularly sensitive to ambient humidity and oxygen levels. Hence, to assess the environmental stability of the system, QS-doped PVA films were cast from a single batch and divided into paired specimens, one of which was encapsulated with a pressure-sensitive laminate, while the other one was left non-laminated. Over 14 days under ambient laboratory conditions, the absorbance and fluorescence of both films remained unchanged, whereas the exhibited phosphorescence diverged significantly. The unlaminated film exhibited a progressive loss of afterglow intensity, a noticeable red shift in the phosphorescence spectrum, and a pronounced shortening of the phosphorescence lifetime, while the laminated film retained its initial RTP intensity, spectral profile, and lifetime throughout the entire experiment.

Optics doi: 10.3390/opt7010006

Authors: Brandon R. Sulvarán-Salmoreno Diego Torres-Armenta Dulce Gonzalez-Utrera David Moreno-Hernández

This work presents a compact refractometric system based on In-Line Digital Holography (ILDH) for the non-invasive characterization of transparent media, encompassing both liquids and high-refractive-index optical glasses. The core of the system is a cost-effective, lensless setup in which a 532 nm laser source and a microscope objective generate a divergent spherical wavefront that illuminates a 10 μm aluminum particle. The resulting diffraction pattern, modulated by samples in the optical path, is recorded by a CMOS sensor. The refractive index of the sample is determined by numerically locating the axial position of the particle-reconstructed image, which directly corresponds to the optical path difference introduced by the test medium. The optimal reconstruction plane is objectively located using an autofocus algorithm based on the Kurtosis metric, which identifies the sharpest image. The system successfully characterizes media across a broad refractive index range from 1.33 to 1.78, yielding linear calibration curves for both liquid and solid samples. The instrument achieves an axial reconstruction resolution of 30 μm and a refractive index precision of ±0.01 RIU. This ILDH approach offers a highly portable, cost-effective, and non-contact solution for refractive index measurement, demonstrating significant potential for industrial quality control and high-throughput point-of-care applications.

Optics doi: 10.3390/opt7010005

Authors: Xintian Song Zhongxin Zhang Reyihanguli Tudi Abulimiti Yasen Mei Xiang Bumaliya Abulimiti

In this work, we synthesized a lead-free halide double perovskite, Cs2CuSbCl6, with high carrier mobility via a one-pot hot-injection method. When combined with a high-resistivity silicon wafer, it forms a Type-II heterojunction structure, and its modulation depth reaches 84% by adjusting the annealing temperature. It demonstrates promising modulation performance at 532 nm. Owing to its strong absorption in the ultraviolet region, Cs2CuSbCl6 shows potential for application in ultraviolet-controlled terahertz modulation.

Optics doi: 10.3390/opt7010004

Authors: Xiongxiong Wu Yanning Yang Zhihui He

Multiscale optical imaging is expected to address the trade-off between field of view (FOV) and resolution in optical systems. To achieve high resolution imaging with a large FOV, this study employs a double-layer monocentric lens to design the front-stage objective lens and utilizes multiple relay lenses for the secondary system. The design results demonstrate that the RMS value of the image spot size across the full FOV is controlled within 2 μm, and the system’s optical modulation transfer function (MTF) across the full FOV approaches the diffraction limit. Specifically, the MTF values across the full FOV exceed 0.35 at the cutoff frequency of 250 lp/mm. The designed optical system features a simple structure and high imaging quality. When a larger number of secondary relay imaging systems are employed, it is capable of achieving a large FOV with high resolution imaging performance, as required by the optical system. Moreover, it holds significant application potential in wide-area, large-range imaging and related fields.

Optics doi: 10.3390/opt7010003

Authors: Ruixin Jia Hongming Fei Han Lin Yibiao Yang Xin Liu Mingda Zhang Liantuan Xiao

Object identification (OI) is widely used in fields like autonomous driving, security, robotics, environmental monitoring, and medical diagnostics. OI using infrared (IR) images provides high visibility in low light for all-day operation compared to visible light. However, the low contrast often causes OI failure in complex scenes with similar target and background temperatures. Therefore, there is a stringent requirement to enhance IR image contrast for accurate OI, and it is ideal to develop a fully automatic process for identifying objects in IR images under any lighting condition, especially in photon-deficient conditions. Here, we demonstrate for the first time a highly accurate automatic IR OI process based on the combination of polarization IR imaging and artificial intelligence (AI) vision (Yolov7), which can quickly identify objects with a high discrimination confidence level (DCL, up to 0.96). In addition, we demonstrate that it is possible to achieve accurate IR OI in complex environments, such as photon-deficient, foggy conditions, and opaque-covered objects with a high DCL. Finally, through training the model, we can identify any object. In this paper, we use a UAV as an example to conduct experiments, further expanding the capabilities of this method. Therefore, our method enables broad OI applications with high all-day performance.

Optics doi: 10.3390/opt7010002

Authors: Hiroki Suto Yugo Minegishi Yasutomo Nomura

Estimating the depth of a fluorescently labeled tumor is beneficial in tumor resection. In this study, we proposed a method for the three-dimensional position estimation of fluorescent tumors using Monte Carlo simulations. A limited proof-of-concept experiment was conducted, and the two-dimensional position of a tumor was estimated by calculating the centroid of the fluorescence distribution, which was obtained by using excitation light to scan the surface of the model. The depth of the tumor was estimated by fitting the analytical equation based on Beer’s law to the diffuse fluorescence profile on the surface of the model. In the estimation of the two-dimensional position, the distance between the embedded and estimated tumor coordinates was 0.71 mm. The estimated tumor depths of 2–6 mm closely matched the embedded depths, with an error rate of approximately 20%. In previous studies, depth estimation was limited to 1–5 mm using visible light, whereas for the simulation used in the present study, the use of longer wavelengths enabled estimation at slightly greater depths.

Optics doi: 10.3390/opt7010001

Authors: Chenxing Liao Huihuang Cai Jiabao Wu Wei Xie Liaolin Zhang

Mn4+-activated fluoride phosphors possess outstanding luminescent properties, making them highly suitable for applications in lighting and display technologies. However, the synthesis of such phosphors generally requires the use of large amounts of highly toxic aqueous HF, leading to serious environmental pollution. To eliminate the use of hazardous HF solution, a low-temperature molten salt method employing NH4HF2 was developed to synthesize the narrow-band red emitter Cs2GeF6: Mn4+ phosphor. Following the reaction, the product was washed with a dilute H2O2 solution to remove residual NH4HF2 and other impurities. The phase purity and morphology were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM), respectively, and the luminescence properties were examined via photoluminescence (PL) spectroscopy. The obtained phosphors exhibit bright red emission characteristics of Mn4+ under blue-violet excitation. Among them, Cs2GeF6: 0.08 Mn4+ shows the highest emission intensity, with an internal quantum efficiency (IQE) of 78%. A white light-emitting diode (WLED) fabricated by combining this phosphor with a blue chip and commercial Y3Al5O12: Ce3+ (YAG) phosphor achieved a high luminous efficacy (LE) of ~146 lm/W, a correlated color temperature (CCT) of ~4396 K, and a color rendering index (Ra) of ~83, alongside excellent operational color stability.

Optics doi: 10.3390/opt6040067

Authors: Zhaoxu Pan Qiaobai He Ruicong Zhang Tianyu Wang Jiaqi Zhu Zicheng Song

With the rapid advancement of detection technologies, traditional static electromagnetic absorbers increasingly struggle to meet controllable stealth requirements across diverse dynamic environments. To achieve active and controllable modulation of electromagnetic reflection characteristics, this paper proposes a transparent reconfigurable metamaterial absorber based on an origami structure. By adjusting the folding angles of the indium tin oxide (ITO)-polyethylene terephthalate (PET) film, the structure achieves reversible deformation from the vertical state to the horizontal state. This enables continuous modulation of the reflectance from below −10 dB (absorbing state) to nearly 0 dB (reflecting state) within the 4–18.9 GHz frequency range, with a relative bandwidth exceeding 130% and excellent angular stability. The energy loss and current distribution under different states are analyzed, revealing the mechanisms behind broadband absorption and deep modulation. Experimental measurements of the fabricated metamaterial align well with simulation results. Leveraging its flexible structure, reversible modulation capability, and angular stability, this origami-inspired reconfigurable metamaterial demonstrates promising application potential in the fields of adaptive electromagnetic camouflage and stealth protection.

Optics doi: 10.3390/opt6040066

Authors: Yu-Han Zhou Guang-Ze Li Lu-Hong Zhang Ning-Chen Cao Li-Ming Zhu Xiao-Bo Hu Yan Wu Khian-Hooi Chew Rui-Pin Chen

Manipulating complex light fields through highly anisotropic scattering medium (HASM) remains a fundamental challenge due to the intricate underlying physics and broad application potential. We introduce a unified theoretical and experimental framework for generating and controlling arbitrarily polarized curved caustic beams using an extended polarization transfer matrix (EPTM) for the first time, enabling intuitive polarization transformation through HASM. The EPTM is experimentally measured via a four-step phase-shifting technique, and its submatrices are independently modulated with tailored caustic phase profiles. This strategy facilitates the creation of diverse high-resolution caustic beams, including Gaussian and vortex types with tunable energy distribution, polarization states, and vorticity. The achievement of polarization transformation through HASM by our approach offers versatile manipulation over optical field properties such as multiple high-resolution caustic beams, angular momentum flux, and polarization, paving the way for enhanced functionality in advanced optical systems.

Optics doi: 10.3390/opt6040065

Authors: Qingran Liu Chenyan Zhang Pengju Hu Huanjie Chen Xiyan Xu Chongfu Zhang

In order to further mitigate the channel non-uniformity at the junction between the input slab and the arrayed waveguide grating in traditional AWG structures, we design a highly flexible, structurally adaptive linear auxiliary waveguide. Through systematic parameter scanning utilizing the Particle Swarm Optimization (PSO) algorithm, an optimal set of geometric parameters for the auxiliary waveguide is identified. This optimization strategy achieves a significant reduction in loss non-uniformity by 0.5 dB relative to the conventional AWG configuration, culminating in a final non-uniformity of merely 0.253 dB. This improvement underscores the critical role of advanced structural tuning and algorithmic optimization in enhancing the performance of photonic integrated circuits, particularly in dense wavelength division multiplexing (DWDM) applications for next-generation communication systems such as radio-over-fiber (RoF) architecture-based 6G. The method can provide a scalable and efficient pathway toward high-uniformity, AWG designs without introducing additional fabrication complexity or incurring substantial costs.

Optics doi: 10.3390/opt6040064

Authors: Quang-Khoi Nguyen Quoc-Cuong Nguyen

An efficient method for evaluating the spectral irradiance properties of the white light of white LEDs is conducted. The method includes two main steps. The first step is to build up spectral irradiance modeling for the blue and yellow emission bands. The photometric parameter of the spectral irradiance of white light which is generated by yellow and blue light mixing is determined based on the photometry and colorimetry theories. The correlated color temperature value strongly depends on the power ratios of blue and yellow light. In addition, the result indicates that the emission bandwidth of yellow phosphor is also an important factor for increasing the color performance of output light. The selection of material with a broader bandwidth of yellow light can control a slower variation in color property compared to the case of using a material with a narrower bandwidth. In addition, the blue light hazard ratio of the spectral irradiance of white light can be extracted, which is helpful for designing the white light with moderate blue and yellow power ratios before fabricating the white LEDs product.

Optics doi: 10.3390/opt6040063

Authors: Mohamed Beriniz Kamal Maaider Noureddine El Barbri Ali Amkor Abdelghani Khalil

This study introduces a new theoretical framework for the ferroelectric phase transition in lithium niobate (LiNbO3), which explicitly incorporates electrostatic interactions between both first and second nearest-neighbor ions. This extended model is applied to estimate the inverse quality factor (Q−1), the equivalent mechanical resistance (Rm), and the Curie temperature (Tc) of pure and titanium-doped lithium niobate (LiNbO3:Ti). The proposed analytical expression for Tc is given by: TC*=2−p*3cos(θ*3)−B*32−p3cos(θ3)−B3 TC. The analysis reveals that variations in Q−1 and Tc are governed by factors such as ionic mass, charge, and defect structure. The theoretical predictions show good agreement with experimental data reported in the literature—particularly for Q−1 in pure LiNbO3 and for Tc in Ti-doped LiNbO3—thus validating the reliability of the proposed model. Moreover, at constant temperature, both the inverse quality factor and the equivalent mechanical resistance decrease as the Ti concentration increases. This trend highlights that titanium doping enhances the acoustic performance of LiNbO3 substrates, making them more suitable for high-performance surface acoustic wave (SAW) device applications.

Optics doi: 10.3390/opt6040062

Authors: Bowei Lv Shitao Li Jie Liu

Optical encoders are high-precision positioning sensors based on the principle of grating diffraction. However, harmonic distortion remains a critical factor limiting the further improvement of measurement accuracy. In response to this challenge, this paper proposes a strategy to suppress harmonic components in the output signals of optical encoders. In this work, a general expression for the light intensity distribution of the grating image is derived. Then, orthogonal sine-cosine signals are generated using a grid photoelectric sensor array, which replaces the conventional slit grating. Furthermore, a method for the co-optimization of the photosensitive unit width and offset is proposed, which effectively suppresses the third and fifth harmonic components. Theoretical and simulation results collectively demonstrate that the proposed method achieves near-complete suppression of the third and fifth harmonics, leading to a significant improvement in output signal quality. This work provides an effective approach for developing high-precision optical encoder systems with low harmonic distortion.

Optics doi: 10.3390/opt6040061

Authors: Shahrzad Dehghani Christopher Knoth Shaghayegh Eskandari Maximilian Buchmüller Tobias Meisen Patrick Görrn

This study presents a data-driven inverse design approach for one-dimensional hybrid waveguide gratings using full reflection spectra across the visible range and a complete span of incident angles. Traditionally, designing such structures to achieve specific optical responses relies on parameter sweeps and iterative simulations which are computationally expensive, time-consuming, and often inefficient. To overcome this, we generated a comprehensive dataset using rigorous coupled-wave analysis (RCWA) simulations and trained two machine learning models: a deterministic tandem network and a generative conditional Variational Autoencoder (cVAE). Both models were trained on noisy reflection spectra to mimic real-world measurements. They both predict structural parameters accurately on clean and noisy data. On clean data, the mean absolute error (MAE) for silver thickness and grating period is below 1 nm. For the dielectric layer, the error is about 13–15 nm. When noise is added, the Tandem network performs best with low to moderate noise. The cVAE, however, stays more stable under high noise conditions. At σ=0.3, the cVAE model reliably predicts the silver thickness and grating period, with MAEs below 6 nm. The main error comes from the dielectric thickness. Sensitivity analysis of reflection spectra confirms this trend. The reflection is least sensitive to the dielectric thickness, while silver thickness and grating period dominate. This analysis provides physical insight for waveguide design as well in which, accurate control of silver thickness and grating period is far more critical than small errors in dielectric thickness. In general, our approach enables rapid prediction of structural parameters of hybrid waveguide gratings from reflection spectra. This reduces design time and reliance on complex microscopic measurements, with potential applications in sensing, communication, and integrated photonics.

Optics doi: 10.3390/opt6040060

Authors: Pedro Pereyra Victor G. Ibarra-Sierra

Earlier quantum calculations of the optical response of Nakamura’s blue laser diode, assuming Kronig–Penney-like band-edge profiles, omitted the effects of charge polarization, cladding-layer asymmetry, and recombination delay times, while such simplified model reproduces the overall emission structure, underestimates the spectral width and fails to capture the decrease in peak intensities at higher energies. Here, we present a detailed quantum theory of polarized-asymmetric superlattices that explicitly incorporates spontaneous and piezoelectric polarization, confining-layer asymmetry, and recombination lifetimes. Local Stark fields are modeled by linear band-edge potentials, and the corresponding Schrödinger equation is solved using Airy functions within the Theory of Finite Periodic Systems. This approach enables the exact calculation of subband eigenvalues, eigenfunctions, transition probabilities and optical spectra. We show that to faithfully reproduce Nakamura’s blue laser spectra, smaller effective masses must be considered, unless unrealistically small barrier heights and widths are assumed. Furthermore, by employing the time distribution of transition probabilities, we capture the energy dependence of recombination lifetimes and their influence on peak intensities. The resulting analysis reproduces the observed spectral broadening and peak-height evolution, while also providing estimates of the magnitude of the Stark effect and mean recombination lifetimes.

Optics doi: 10.3390/opt6040059

Authors: Dustin Louisos Joseph Engeland Nuren Z. Shuchi Samuel I. Gatley John F. Federici Benjamin Thomas Ian Gatley Glenn D. Boreman Tino Hofmann

This work focuses on the characterization of the complex dielectric function of a polymer material, which is UV-cured dielectric ink 1092, used in the DragonFly IV 3D inkjet printer. Infrared spectroscopic ellipsometry was performed over the spectral range of 300–4000 cm−1 at multiple angles of incidence to extract both real and imaginary components of the dielectric response. In addition, polarized transmission measurements were taken over the spectral range from 300–6000 cm−1 to aid in characterization. We report an isotropic dielectric function model that is composed of oscillators with both Gaussian and Lorentzian broadening. This model reveals strong absorption bands at 925–1500 cm−1, 1600–1775cm−1, and 2840–3000 cm−1 while otherwise appearing largely transparent. This parameterized dielectric function is critical in first-principles modeling of infrared optical components and metamaterials fabricated using this polymer.

Optics doi: 10.3390/opt6040058

Authors: Pedro Marcos Velasco-Bolom Jorge Luis Camas-Anzueto Rocío Meza-Gordillo Madaín Pérez-Patricio Marcoantonio Ramírez-Morales Gilberto Anzueto-Sánchez Rubén Grajales-Coutiño José Antonio Hoyo-Montaño

The scientific community has been interested in lophine’s versatility and usage in various applications. Research has shown that humic acid is a material that exhibits interference with lophine. Humic molecules associate with each other in supramolecular conformations through weak hydrophobic interactions at alkaline or neutral pH and hydrogen bonds at low pH. This work presents the characterization of a sensitive lophine layer based on water’s hydrogen ions (pH). We conducted a spectroscopy study to analyze how the absorbance at different amounts of lophine depends on pH. This study demonstrates the hyperchromic behavior of imidazole at various pH values, which may be utilized in an intrinsic fiber optic pH sensor. The dynamic range of the fiber optic sensor was 5 to 11.3 pH units. The sensor was developed by coating a thinned fiber with a sensitive lophine layer. It achieves a sensitivity of 0.27 dB/pH and a response time of 5 s.

Optics doi: 10.3390/opt6040057

Authors: Renan Carvalho Diego Pinheiro Henrique Dinarte Raul Almeida Carmelo Bastos-Filho

To meet the increasing demands for data, elastic optical networks (EONs) require highly efficient resource management. While classical Routing and Spectrum Assignment (RSA) algorithms establish a path and allocate spectrum, advanced versions such as Routing, Modulation-format-selection, and Spectrum Assignment (RMSA) also optimize modulation format selection. However, these approaches often lack adaptability to diverse network aspects. The hybrid routing and spectrum assignment (HRSA) algorithm offers a more flexible and robust approach by providing multiple choices between route (resource savings) and spectrum prioritization (fragmentation mitigation and network load balancing) for each network node pair. Despite its potential, the adaptive nature of HRSA introduces complexity, and the influence of topological features on its decisions remains not fully understood. This knowledge gap hinders the ability to optimize network design and resource allocation fully. This paper examines how topological features influence HRSA’s adaptive decisions regarding routing and spectrum assignment prioritization for source-destination node pairs in EONs. By employing machine learning approaches—Decision Tree (DT), Random Forest (RF), Extreme Gradient Boosting (XGBoost), and Support Vector Machine (SVM)—we model and identify the key topological features that influence HRSA’s decision-making. Then, we compare the models generated by each approach and extract insights using an a posteriori analysis technique to evaluate feature importance. Our results show the algorithm’s behavior is highly predictable (over 91% accuracy), with decisions driven primarily by the network’s structure and node metrics. This work advances the understanding of how topological features influence the RSA problem.

Optics doi: 10.3390/opt6040056

Authors: Mamoun M. Bader Phuong-Truc T. Pham Juri A. Busaili Samar M. Alrifai Sarah H. Younas El Hadj Elandaloussi

Dicyanovinyl (DCV) oligothiophenes are interesting materials due to their unique optical and electronic properties. They are relatively easy to prepare using Knoevenagel condensation reactions from the corresponding aldehydes. Understanding their optical and electrochemical characteristics is important for both building structure/property relationships and for optimizing their performance in various applications. We report on the electrochemical and photophysical properties of three oligothiophenes end-capped with dicyanovinyl -CH=C(CN)2 or DCV groups. The compounds included in this study are DCV-T-DCV (1), DCV-2T-DCV (2), and DCV-3T-DCV (3), where T represents one thiophene unit. Introduction of the DCV groups into oligothiophenes results in unique evolution of their electrochemical and optical behavior. First, new reversible two-electron reduction processes in the series DCV-nT-DCV start to appear with a gradual increase in the reduction potential with an increasing number of thiophene units. This was consistent with the electronic spectroscopic results. These results demonstrate that the DCV groups can be used in molecular design and fine-tuning of the optical and redox properties of oligothiophene and presumably this strategy can be extended to other conjugated organic molecules. We also report on the photophysical and vibrational spectroscopic properties of these compounds. The C=C stretching bands in Raman and IR spectra reveal more quinoidal nature in shorter molecules and more dominant benzoidal character in longer molecules. The DCV-induced modulation of electrochemical, optical, and vibrational properties highlights their potential in diverse optoelectronic applications.

Optics doi: 10.3390/opt6040055

Authors: Tong Wu Haopeng Li Chuan Zhang Jingwei Yu Jianjun Liu Zepei Zheng Bosong Duan Anyu Sun Bingfeng Ju

With the increasing application of oxide films in nuclear fuel assemblies, the accurate measurement of thin-film thickness has become increasingly critical. Traditional spectral interferometry techniques have limitations when dealing with new materials and complex structures; therefore, this study proposes a multi-channel wide-spectrum high-resolution analysis technique. This technique optimizes the utilization of photosensitive elements through multi-channel spectral sampling, combined with precision spectroscopic components and an independent optical focusing and imaging system. Simultaneously, it adopts optical correction technologies such as coma optimization and astigmatism correction to improve imaging quality and spectral resolution. Additionally, it enhances data accuracy by means of multi-channel calibration based on the least squares method and non-linear correction. The technique enables high-precision measurement ranging from the nanometer to the millimeter scale, resulting in a significantly wider measurement range compared to traditional spectrometers. Simulation verification shows that this technique outperforms existing technologies in information acquisition, analysis accuracy, and detection efficiency, and has broad application prospects in fields such as semiconductor chip manufacturing and optical coating. In the future, focus will be placed on expanding the spectral range, improving resolution, and enhancing real-time measurement capabilities.

Optics doi: 10.3390/opt6040054

Authors: Fan Li Junliang Yang Hui Zhang Pingquan Wang

Digital image correlation (DIC) technology is widely employed in speckle-based measurement techniques, including X-ray speckle tracking. By enhancing DIC’s measurement capability to the subpixel scale through subpixel registration technology, the accuracy of such tracking methods is significantly improved. Consequently, selecting an appropriate subpixel registration algorithm becomes crucial for advancing the precision of both DIC and its application in tracking methods. Nevertheless, current evaluation approaches for these algorithms overlook spatial resolution—an essential metric not only for X-ray speckle tracking but also for other comparable methodologies. Inspired by the modulation transfer function, this study proposes a novel evaluation method that uses the root mean square error of displacement measurement at different spatial frequencies to assess spatial resolution. Two widely used subpixel registration algorithms—the peak-finding algorithm and the iterative spatial domain cross-correlation algorithm—are evaluated and compared. The result strongly contrasts with traditional evaluations based on ideal translational conditions, and underscores the necessity of incorporating spatial resolution and speckle size into algorithm selection criteria for practical applications.

Optics doi: 10.3390/opt6040053

Authors: Roman V. Cherbunin Aleksey Liubomirov Stella V. Kavokina Denis Novokreschenov Andrey Kudlis Alexey V. Kavokin

We experimentally observe harmonic oscillations in a bosonic condensate of exciton-polaritons confined within an elliptical trap. These oscillations arise from quantum beats between two size-quantized states of the condensate, split in energy due to the trap’s ellipticity. By precisely targeting specific spots inside the trap with nonresonant laser pulses, we control frequency, amplitude, and phase of these quantum beats. The condensate wave function dynamics is visualized on a streak camera and mapped to the Bloch sphere, demonstrating Hadamard and Pauli-Z operations. We conclude that a qubit based on a superposition of these two polariton states would exhibit a coherence time exceeding the lifetime of an individual exciton-polariton by at least two orders of magnitude.

Optics doi: 10.3390/opt6040052

Authors: Weihang Xing Xuquan Wang Zhiyuan Ma Yujie Xing Xiong Dun Xinbin Cheng

Platycodonis radix is a commonly used traditional Chinese medicine (TCM) material. Its bioactive compounds and medicinal value are closely related to its geographical origin. The internal components of Platycodonis radix from different origins are different due to the influence of environmental factors such as soil and climate. These differences can affect the medicinal value. Therefore, accurate identification of Platycodonis radix origin is crucial for drug safety and scientific research. Traditional methods of identification of TCM materials, such as morphological identification and physicochemical analysis, cannot meet the efficiency requirements. Although emerging technologies such as computer vision and spectroscopy can achieve rapid detection, their accuracy in identifying the origin of Platycodonis radix is limited when relying solely on RGB images or spectral features. To solve this problem, we aim to develop a rapid, non-destructive, and accurate method for origin identification of Platycodonis radix using hyperspectral imaging (HSI) combined with deep learning. We captured hyperspectral images of Platycodonis radix slices in 400–1000 nm range, and proposed a deep learning classification model based on these images. Our model uses one-dimensional (1D) convolution kernels to extract spectral features and two-dimensional (2D) convolution kernels to extract spatial features, fully utilizing the hyperspectral data. The average accuracy has reached 96.2%, significantly better than that of 49.0% based on RGB images and 81.8% based on spectral features in 400–1000 nm range. Furthermore, based on hyperspectral images, our model’s accuracy is 14.6%, 8.4%, and 9.6% higher than the variants of VGG, ResNet, and GoogLeNet, respectively. These results not only demonstrate the advantages of HSI in identifying the origin of Platycodonis radix, but also demonstrate the advantages of combining 1D convolution and 2D convolution in hyperspectral image classification.

Optics doi: 10.3390/opt6040051

Authors: Weiyu Cai Guangwang Ding Xiaomei Liu Xiang Li Houjie Chen Xiaojuan Ma Hua Liu

The complex aquatic environment attenuates light transmission, thereby limiting the detection range of underwater laser systems. To address the challenges of limited operational distance and significant light energy attenuation, this study investigates optimized underwater lighting and imaging applications using a combined tricolor RGB (RED-GREEN-BLUE) white laser source. First, accounting for the attenuation characteristics of water, we propose a power-compensated white laser system based on transmission distance and underwater imaging theory. Second, underwater experiments are conducted utilizing both standard D65 white lasers and the proposed power-compensated white lasers, respectively. Finally, the theory is validated by assessing image quality metrics of the captured underwater imagery. The results demonstrate that a low-power (0.518 W) power-compensated white laser achieves a transmission distance of 5 m, meeting the requirements for a long-range, low-power imaging light source. Its capability for independent adjustment of the three-color power output fulfills the lighting demands for specific long-distance transmission scenarios. These findings confirm the advantages of power-compensated white lasers in long-range underwater detection and refine the characterization of white light for underwater illumination.

Optics doi: 10.3390/opt6040050

Authors: Toshihiko Ogura

The optical microscope is an indispensable observation instrument that has fundamentally contributed to progress in science and technology. Dark-field microscopy and scattered light imaging techniques enable high-contrast observation of nanoparticles in water. However, the scattered light is focused by the optical lenses, resulting in a blurred image of the nanoparticle structure. Here, we developed a projection optical microscope (PROM), which directly observes the scattered light from the nanoparticles without optical lenses. In this method, the sample is placed below the focus position of the microscope’s objective lens and the projected light is detected by an image sensor. This enables direct observation of the sample with a spatial resolution of approximately 20 nm. Using this method, changes in the aggregation state of nanoparticles in solution can be observed at a speed faster than the video frame rate. Moreover, the mechanism of such high-resolution observation may be related to the quantum properties of light, making it an interesting phenomenon from the perspective of optical engineering. We expect this method to be applicable to the observation and analysis of samples in materials science, biology and applied physics, and thus to contribute to a wide range of scientific, technological and industrial fields.

Optics doi: 10.3390/opt6040049

Authors: Yueying Li Jiazhu Duan Xiangjie Zhao Yingnan Peng Yongquan Luo Dayong Zhang Yibo Chen

The Volume Bragg Grating (VBG) imaging technique provides a novel approach to gaze-type hyperspectral imaging. However, collimation constraints of the incident beam during narrow-band filtering and high-spatial-resolution imaging introduce system complexity, hindering miniaturization and modularization of the optical system. To address these limitations, this paper proposes a convex transmissive VBG structure with tunable design parameters to enhance the field of view (FOV), relax collimation requirements, improve imaging quality, narrow filter spectral bandwidth, and simplify the optical system design. For the precise analysis and optimization of convex VBG performance, we established a physical model for filtered imaging using a convex transmissive VBG with polychromatic extended sources. An evaluation metric termed the “Maximal Splitting Angle (MSA)” was introduced to quantify the dispersion extent of image spots. This approach was employed to investigate the intrinsic correlations between structural parameters (such as the radius of curvature, vector tilt angle, grating period, and thickness) and key system performance indicators (spatial resolution and spectral resolution). The necessity of optimizing these parameters was rigorously demonstrated. Theoretical analysis confirms that convex transmissive VBG achieves superior spatial and spectral resolution over planar VBG under reduced collimation constraints. The experimental results show a 58.5% enhancement in spatial resolution and a 63.6% improvement in spectral bandwidth for the convex transmissive VBG system. Crucially, while planar transmissive VBG suffers from stray fringe interference during wavelength tuning, its convex counterpart remains unaffected. This study proposes a novel device structure, offering new perspectives for optimizing VBG-filtered spectral imaging systems.

Optics doi: 10.3390/opt6040048

Authors: Zhixiao Xu Hangbo Hua Jing Yu Zhitian Niu Ming Kong

Traditionally, retrieving both temperature and CO2 concentration in atmospheric remote sensing has relied on two independent lidar systems, leading to increased system complexity and limited coordination. To address this challenge, we propose a coordinated retrieval approach using a three-wavelength differential absorption lidar (DIAL) system. A temperature-sensitive wavelength is selected to distinguish strong absorption from weak absorption, forming the tri-wavelength configuration. By exploiting the different sensitivities of absorption cross-sections to thermal and molecular variations, simultaneous retrieval of both parameters is achieved. A standard atmospheric profile under clean-air conditions is constructed. The CO2 absorption spectrum near 1573 nm is generated using Voigt line shapes and data from the HITRAN database. Extinction and backscatter coefficients are retrieved through the Klett method. A layer-by-layer solution of the coupled differential equations is then performed to extract temperature and concentration simultaneously. Results are benchmarked against the atmospheric model, demonstrating the feasibility of the approach. This method provides a promising pathway for high-precision, multi-parameter DIAL sensing.

Optics doi: 10.3390/opt6040047

Authors: Yingying Li Zhuang Liu Shuwan Yu Qiang Fu Yingchao Li Chao Wang Haodong Shi

Wireless optical communication technology offers advantages, such as high-data transmission rates, confidentiality, and robust anti-interception capabilities, making it highly promising for cross-sea–air interface communication applications. However, to our knowledge, no studies have been conducted on the spatial transmission efficiency of light after it passes through ocean waves. To address this issue, a seawater-wave–atmosphere model based on Gerstner waves was constructed. Using the Monte Carlo method, the optical power distributions of the laser light passing through the sea–air interface at the first- and second-level sea scales were simulated. The optimal positions for deploying one to three receiving optical systems were analyzed, and a laser communication receiving system was designed. Furthermore, simulations were conducted to determine the optical transmission efficiency of the designed optical receiver system. At the first-level sea scale, the optimal position for a single-point detector was (0°, ±5.61°), whereas those for the two detectors were (0°, ±5.61°) and (0°, ±5.68°). At the second-level sea scale, the optimal position for a single-point detector was (0°, ±3.17°), and the optimal positions for the two detectors were (0°, ±3.1°) and (0°, ±2.98°). Under the designed conditions, the optical transmission efficiency for a single detector at the first- and second-level sea scales was 0.74–0.88%, respectively, while it was 0.79–1.09% in the two-detector case. At the second-level sea scale, the optical transmission efficiency for a single detector was 0.37–2.09% and 0.50–1.97% in the two-detector case.

Optics doi: 10.3390/opt6040046

Authors: Dulce Araceli Guzman-Rocha Alejandrina Martinez-Gamez José Luis Lucio-Martinez Carlos Herman Wiechers-Medina Mario Eduardo Cano-Gonzales Rene Garcia-Contreras

The use of magnetic materials, such as ferrofluids, is of great importance in biomedical applications, and as a result, interest in studying their magneto-optical properties has grown significantly in recent years. Therefore, in this work, magnetic nanoparticles were synthesized with chitosan coating, leaving the product as a ferrofluid in aqueous solution. Structural, morphological, magnetic, and optical characterization was carried out obtaining a cubic structure centered on the faces, a spherical morphology with a size distribution of 10–14 nm according to TEM images and a magnetic saturation of 53 emu/g. In the optical properties, the effect of chitosan shell on the forbidden band was studied, showing a blue-shifting effect, due to reduction on the inner magnetic nanoparticles size.

Optics doi: 10.3390/opt6040045

Authors: Desta Regassa Golja Megersa Olumana Dinka

Zinc oxide (ZnO), a wide-bandgap semiconductor, has garnered significant interest for photocatalytic applications due to its excellent chemical stability, non-toxicity, and strong oxidative capability. In this study, density functional theory (DFT) calculations were employed to explore the impact of ruthenium (Ru) doping on the structural, electronic, and magnetic properties of wurtzite ZnO. The introduction of Ru leads to bandgap narrowing and the emergence of impurity states, thereby enhancing visible light absorption. Charge density analysis reveals enhanced electron delocalization, while the projected density of states (PDOS) indicates strong hybridization between the Ru 4d orbitals and the ZnO electronic states. The density of states at the Fermi level, N(EF), exhibits a notable dependence on doping concentration and magnetic configuration. For non-magnetic states, N(EF) reaches 11 states/eV and 9.5 states/eV at 12.5% and 25% Ru concentrations, respectively. In ferromagnetic configurations, these values decrease to 0.65 states/eV and 1.955 states/eV, while antiferromagnetic states yield 4.945 states/eV and 0.65 states/eV. These variations highlight Ru’s crucial role in regulating electronic density, thereby affecting electrical conductivity, magnetic properties, and photocatalytic efficiency. The results offer theoretical guidance for designing high-performance Ru-doped ZnO photocatalysts.

Optics doi: 10.3390/opt6030044

Authors: Shuai Zhou Jiwen Xu Liang Zhao Yuqi Yang Zengqiang Li Junjie Zhang

While laser-assisted turning (LAT) improves the machinability of GH 4169 through heating-induced thermal softening, revealing the influence of the laser processing parameters on its thermal field and machining efficiency is crucial. In this study, the influence of different laser processing parameters on the thermal field during the preheating process of LAT is systematically investigated by combining finite element (FE) simulation and experimentation, from which the optimal processing parameters of the LAT of GH 4169 are obtained. Firstly, the experimental platform of LAT is established, and a 2D FE model of the LAT of GH 4169 is constructed. Secondly, the absorption coefficient of GH 4169 with a 1064 nm wavelength laser is calibrated through experimentation and FE simulation, which lay an accurate foundation for the subsequent thermal field analysis. Furthermore, the FE simulation of the preheating process of the LAT of GH 4169 is carried out, focusing on the influence of laser power, laser spot diameter, laser spot movement speed and laser spot–tool edge distance on the thermal field, in terms of the peak and final preheating temperatures. The results show that laser power, laser spot movement speed and laser spot diameter have a significant influence on both of the two temperatures, while laser spot–tool edge distance only affects the final preheating temperature. In addition, the regression equations of the peak and final preheating temperatures are obtained based on the FE simulation results, and the optimal processing parameters are determined by combining the boundary conditions (peak temperature of 650–950 °C and initial preheating temperature of ≤190 °C). Comparison experiments with conventional turning (CT) show that under the optimal processing parameters, LAT can effectively reduce the cutting force, surface roughness and tool flank wear, which indicates that a rational selection of laser processing parameters is crucial for improving the capability of LAT of GH 4169.

Optics doi: 10.3390/opt6030043

Authors: Xinyan Lin Yichan Zhang Yinglong Kang Sheng Li Qiuming Nan Lina Yue Yan Yang Min Zhou

To address the challenge of efficiently identifying and providing early warnings for typical structural damages in small and medium-sized bridges during long-term service, this paper proposes an intelligent monitoring and recognition method based on ultra-weak fiber Bragg grating (UWFBG) array sensing. By deploying UWFBG strain-sensing cables across the bridge, the system enables continuous acquisition and spatial analysis of multi-point strain data. Based on this, a series of experimental scenarios simulating typical structural damages—such as single-slab loading, eccentric loading, and bearing detachment—are designed to systematically analyze strain evolution patterns before and after damage occurrence. While strain distribution maps allow for visual identification of some typical damages, the approach remains limited by reliance on manual interpretation, low recognition efficiency, and weak detection capability for atypical damages. To overcome these limitations, machine learning algorithms are further introduced to extract features from strain data and perform pattern recognition, enabling the construction of an automated damage identification model. This approach enhances both the accuracy and robustness of damage recognition, achieving rapid classification and intelligent diagnosis of structural conditions. The results demonstrate that the integration of the monitoring system with intelligent recognition algorithms effectively distinguishes different types of damage and shows promising potential for engineering applications.

Optics doi: 10.3390/opt6030042

Authors: Andreea-Alexandra-Mihaela Muşat Cãlin-Petru Tãtaru Gabriela-Cornelia Muşat Lucia Bubulac Mihai-Alexandru Preda Ovidiu Muşat

Background: This narrative review aims to assess multiple strategies available to evaluate and manage corneal astigmatism in the context of cataract surgery, with a focus on the surgical techniques, intraocular lens (IOL) selection, and the integration of advanced new technologies. Methods: A narrative review based on a literature search in PubMed/MEDLINE and the Cochrane Library, covering publications from 1990 to 2025, was conducted. Eligible studies included randomized controlled trials, observational studies, prospective and retrospective analyses, and systematic reviews. Key search terms included “astigmatism”, “cataract surgery”, “keratometry”, and “refraction.” Studies were screened and selected by two independent reviewers. Results: Corneal astigmatism is the most common form of astigmatism. While the anterior corneal astigmatism plays a more important role, the posterior corneal astigmatism and the posterior-to-anterior corneal ratio (Gullstrand ratio) can impact the postoperative refractive results in a very important way. While planning the cataract surgery, surgically induced astigmatism (SIA), especially on the posterior cornea, must be taken into consideration. Various approaches, such as opposite clear corneal incisions (OCCIs), toric intraocular lens (IOLs), intraoperative aberrometry, and the integration of artificial intelligence and robotic-assisted surgery, are increasing the precision of astigmatism correction and surgical outcomes. Conclusions: Individualized surgical planning and precise measurement are key factors in reducing residual astigmatism and obtaining the best visual outcomes in patients with corneal astigmatism undergoing cataract surgery. By taking into consideration the posterior corneal data, refining IOL calculations, and embracing the rapidly developing technological innovations, patient satisfaction and visual quality can be substantially improved, and the predictability of the surgical outcome can be enhanced.

Optics doi: 10.3390/opt6030041

Authors: Chengtao Liu Peng Song Junxiao Yang Xiaojun Zou

This study explores the transmission characteristics between the links of UV(ultraviolet)-network communication under mobile conditions. Utilizing the prevalent UV-network communication network topology as a foundation, a UV-network communication model tailored to mobile scenarios was developed. This model includes a method for calculating the impulse response of the system, focusing specifically on three common network topology structures: two parallel links, co-address of the originating link, and co-address of the receiving link. The simulation and analysis conducted in this study examine the impact of various factors on the system’s impulse response, such as receiver movement speed, geometric parameters of the receivers, link spacing, and the angle between links. The results indicate that receiver movement speed significantly influences pulse response fading, with faster speeds resulting in more severe fading. Additionally, in parallel links, smaller link spacing results in stronger impulse response. Furthermore, a smaller angle between the originating and receiving co-addresses results in increased inter-link interference. The study findings in this paper will lay the foundation for the study of UV mobile self-organizing networks.

Optics doi: 10.3390/opt6030040

Authors: Anton Krivosheev Dmitriy Kambur Artem Turov Max Belokrylov Yuri Konstantinov Timur Agliullin Konstantin Lipatnikov Fedor Barkov

Optical frequency domain reflectometry (OFDR) is one of the key diagnostic tools for fiber optic components and circuits built on them. A low signal-to-noise ratio, resulting from the low intensity of backscattered signals, prevents the correct quantitative description of the medium parameters. Known methods of signal denoising, such as empirical mode decomposition, frequency filtering, and activation function dynamic averaging, make the signal smoother but introduce errors into its dynamic characteristics, changing the intensity of reflection peaks and distorting the backscattering level. We propose a method to reduce OFDR trace noise using elliptical arc fitting (EAF). The obtained results indicate that this algorithm efficiently processes both areas with and without contrasting back reflections, with zero distortion of Fresnel reflection peaks, and with zero attenuation error in regions without Fresnel reflections. At the same time, other methods distort reflection peaks by 14.2–42.6% and shift the correct level of Rayleigh scattering by 27.2–67.3%. Further work will be aimed at increasing the accuracy of the method and testing it with other types of data.

Optics doi: 10.3390/opt6030039

Authors: Artur Czerwinski

This paper explores the formal structure and philosophical implications of polarization-path entanglement in quantum optics, where different degrees of freedom of a single photon become entangled. We examine the mathematical conditions under which coherence is preserved or lost, emphasizing the role of distinguishability and information flow. The analysis is situated within major interpretational frameworks (including Copenhagen, Many-Worlds, QBism, and Bohmian mechanics) to evaluate whether such entanglement reflects physical reality or epistemic constraints. Finally, we discuss experimental realizations, relevance to quantum information processing, and open conceptual questions regarding the ontological status of single-particle entanglement.

Optics doi: 10.3390/opt6030038

Authors: Junaid Zafar Faisal Sharif Haroon Zafar

Deep neural networks have led to a substantial increase in multifaceted classification tasks by making use of large-scale and diverse annotated datasets. However, diverse optical coherence tomography (OCT) datasets in cardiovascular imaging remain an uphill task. This research focuses on improving the diversity and generalization ability of augmentation architectures while maintaining the baseline classification accuracy for coronary atrial plaques using a novel dual-generator and dynamically fused discriminator conditional generative adversarial network (DGDFGAN). Our method is demonstrated on an augmented OCT dataset with 6900 images. With dual generators, our network provides the diverse outputs for the same input condition, as each generator acts as a regulator for the other. In our model, this mutual regularization enhances the ability of both generators to generalize better across different features. The fusion discriminators use one discriminator for classification purposes, hence avoiding the need for a separate deep architecture. A loss function, including the SSIM loss and FID scores, confirms that perfect synthetic OCT image aliases are created. We optimize our model via the gray wolf optimizer during model training. Furthermore, an inter-comparison and recorded SSID loss of 0.9542 ± 0.008 and a FID score of 7 are suggestive of better diversity and generation characteristics that outperform the performance of leading GAN architectures. We trust that our approach is practically viable and thus assists professionals in informed decision making in clinical settings.

Optics doi: 10.3390/opt6030037

Authors: Lei Huang Yinze Wang

Anti-resonant hollow-core fibers have exhibited excellent performance in applications such as high-power pulse transmission, network communication, space exploration, and precise sensing. Employing anti-resonant hollow-core fibers instead of light guiding arms for transmitting laser energy at the 2.79 μm band can significantly enhance the flexibility of medical laser handles, reduce system complexity, and increase laser transmission efficiency. Nevertheless, common anti-resonant hollow-core fibers do not have the ability to maintain the polarization state of light during laser transmission, which greatly affects their practical applications. In this paper, we propose a polarization-maintaining anti-resonant hollow-core fiber applicable for transmission at the mid-infrared 2.79 μm band. This fiber features a symmetrical geometric structure and an asymmetric refractive index cladding composed of quartz and a type of mid-infrared glass with a higher refractive index. Through optimizing the fiber structure at the wavelength scale, single-polarization transmission can be achieved at the 2.79 μm wavelength, with a polarization extinction ratio exceeding 1.01 × 105, indicating its stable polarization-maintaining performance. Simultaneously, it possesses low-loss transmission characteristics, with the loss in the x-polarized fundamental mode being less than 9.8 × 10−3 dB/m at the 2.79 µm wavelength. This polarization-maintaining anti-resonant hollow-core fiber provides a more reliable option for the light guiding system of the 2.79 μm Er; Cr: YSGG laser therapy device.

Optics doi: 10.3390/opt6030036

Authors: Dillan Cunha Amaral Pedro Lucas Machado Magalhães Alex Gonçalves Sá Alexandre Batista da Costa Neto Flávio Moura Travassos de Medeiros Milton Ruiz Alves Jaime Guedes Ricardo Noguera Louzada

Functional Optical Balance (FOB) is a novel personalized strategy for intraocular lens (IOL) selection in cataract surgery, designed to reconcile the trade-off between optical quality and spectacle independence. FOB is a core concept aiming to maximize visual performance by treating the two eyes as a synergistic pair. One eye (often the dominant eye) is optimized for pristine optical quality (typically distance vision with a high-contrast monofocal or low-add IOL). In contrast, the fellow eye is optimized for extended depth of focus and pseudoaccommodation (using an extended depth-of-focus or multifocal/trifocal IOL) to reduce dependence on glasses. This review introduces the rationale and theoretical basis for FOB, including the interplay of depth of focus and optical aberrations, binocular summation, ocular dominance, and neuroadaptation. We discuss the clinical implementation of FOB: how the first-eye results guide the second-eye IOL choice in a tailored “mix-and-match” approach, as well as practical workflow considerations such as patient selection, ocular measurements, and decision algorithms. We also review current evidence from the literature on asymmetric IOL combinations (e.g., monofocal plus multifocal, or EDOF plus trifocal), highlighting visual outcomes, patient satisfaction, and remaining evidence gaps. Overall, FOB represents a paradigm shift toward binocular, patient-customized refractive planning. Early clinical reports suggest it can deliver a continuous range of vision without significantly compromising visual quality, though careful patient counseling and case selection are essential. Future directions include the integration of advanced diagnostics, artificial intelligence-driven IOL planning tools, and adaptive optics simulations to refine this personalized approach further. The promise of FOB is to improve cataract surgery outcomes by achieving an optimal balance: one that provides each patient with excellent visual quality and functional vision across distances, tailored to their lifestyle and expectations.

Optics doi: 10.3390/opt6030035

Authors: Hanyue Wei Yingying Luo Feiya Ma Liyong Ren

Tongue cancer, the most aggressive subtype of oral cancer, presents critical challenges due to the limited number of specialists available and the time-consuming nature of conventional histopathological diagnosis. To address these issues, we developed an intelligent diagnostic system integrating Mueller matrix microscopy with deep learning to enhance diagnostic accuracy and efficiency. Through Mueller matrix polar decomposition and transformation, micro-polarization feature parameter images were extracted from tongue cancer tissues, and purity parameter images were generated by calculating the purity of the Mueller matrices. A multi-stage feature dataset of Mueller matrix parameter images was constructed using histopathological samples of tongue cancer tissues with varying stages. Based on this dataset, the clinical potential of Mueller matrix microscopy was preliminarily validated for histopathological diagnosis of tongue cancer. Four mainstream medical image classification networks—AlexNet, ResNet50, DenseNet121 and VGGNet16—were employed to quantitatively evaluate the classification performance for tongue cancer stages. DenseNet121 achieved the highest classification accuracy of 98.48%, demonstrating its potential as a robust framework for rapid and accurate intelligent diagnosis of tongue cancer.

Optics doi: 10.3390/opt6030034

Authors: Yazan Alkhlefat Amr M. Ragheb Maged A. Esmail Sevia M. Idrus Farabi M. Iqbal Saleh A. Alshebeili

Terahertz (THz) frequencies, spanning from 0.1 to 1 THz, are poised to play a pivotal role in the development of future 6G wireless communication systems. These systems aim to utilize photonic technologies to enable ultra-high data rates—on the order of terabits per second—while maintaining low latency and high efficiency. In this work, we present a novel photonic method for generating sub-THz vector signals within the THz band, employing a semiconductor optical amplifier (SOA) and phase modulator (PM) to create an optical frequency comb, combined with in-phase and quadrature (IQ) modulation techniques. We demonstrate, both through simulation and experimental setup, the generation and successful transmission of a 0.1 THz vector. The process involves driving the PM with a 12.5 GHz radio frequency signal to produce the optical comb; then, heterodyne beating in a uni-traveling carrier photodiode (UTC-PD) generates the 0.1 THz radio frequency signal. This signal is transmitted over distances of up to 30 km using single-mode fiber. The resulting 0.1 THz electrical vector signal, modulated with quadrature phase shift keying (QPSK), achieves a bit error ratio (BER) below the hard-decision forward error correction (HD-FEC) threshold of 3.8 × 10−3. To the best of our knowledge, this is the first experimental demonstration of a 0.1 THz photonic vector THz wave based on an SOA and a simple PM-driven optical frequency comb.

Optics doi: 10.3390/opt6030033

Authors: Si-Yuan Ma Ying Wang Yuriy I. Mazur Morgan E. Ware Gregory J. Salamo Bao Lai Liang

The optical properties of a heterostructure containing GaSb/GaAs quantum dots (QDs) have been systematically investigated via photoluminescence (PL) measurements to gain insights into carrier dynamics. The QD and wetting layer (WL) emissions exhibit a complementary dependence on the excitation intensity and temperature, reflecting the interplay between carrier localization in the WL and carrier relaxation from the WL to the QDs. Carrier dynamics related to localization, injection, and recombination are further validated by time-resolved photoluminescence (TRPL). These findings highlight the necessity of carefully optimizing GaSb/GaAs QD structures to mitigate the impact of carrier localization, thereby enhancing the ultimate performance of devices utilizing these QDs as active region materials.

Optics doi: 10.3390/opt6030032

Authors: Senfar Wen Yu-Che Wen

Quadcolor cameras with conventional RGB channels were studied. The fourth channel was designed to improve the estimation of the spectral reflectance and chromaticity from the camera signals. The RGB channels of the quadcolor cameras considered were assumed to be the same as those of the Nikon D5100 camera. The fourth channel was assumed to be a silicon sensor with an optical filter (band-pass filter or notch filter). The optical filter was optimized to minimize a cost function consisting of the spectral reflectance error and the weighted chromaticity error, where the weighting factor controls the contribution of the chromaticity error. The study found that using a notch filter is more effective than a band-pass filter in reducing both the mean reflectance error and the chromaticity error. The reason is that the notch filter (1) improves the fit of the quadcolor camera sensitivities to the color matching functions and (2) provides sensitivity in the wavelength region where the sensitivities of RGB channels are small. Munsell color chips under illuminant D65 were used as samples. Compared with the case without the filter, the mean spectral reflectance rms error and the mean color difference (ΔE00) using the quadcolor camera with the optimized notch filter reduced from 0.00928 and 0.3062 to 0.0078 and 0.2085, respectively; compared with the case of using the D5100 camera, these two mean metrics reduced by 56.3%.

Optics doi: 10.3390/opt6030031

Authors: Geovana Lira Santana Raphael Barbosa Vinicius Tribuzi Filippo Ghiglieno Edgar Dutra Zanotto Lino Misoguti Paulo Henrique Dias Ferreira

Chemically strengthened glass is widely used for its remarkable fracture strength, mechanical performance, and scratch resistance. Assessing its hardness is crucial to evaluating improvements from chemical tempering. However, conventional methods like Vickers hardness tests are destructive, altering the sample surface. This study presents a novel, rapid, and nondestructive testing (NDT) approach by correlating the nonlinear refractive index (n2) with surface hardness. Using ultrafast laser pulses, we measured the n2 cross-section via the nonlinear ellipse rotation (NER) signal in Gorilla®-type glass subjected to ion exchange (Na+ by K+). A microscope objective lens provided a penetration resolution of ≈5.5 μm, enabling a localized NER signal analysis. We demonstrate a correlation between the NER signal and hardness, offering a promising pathway for advanced, noninvasive characterization. This approach provides a reliable alternative to traditional destructive techniques, with potential applications in industrial quality control and material science research.

Optics doi: 10.3390/opt6030030

Authors: Samuel Nlend Sune Von Solms Johann Meyer

This paper suggests a perspective-controlled solution for an integrated Infrared micro-/nanoplastic spectroscopy/photometry-based detection, from the diffraction up to the geometry etendue, with the aim of yielding a universal spectrometer/photometer. Spectrophotometry, unlike spectroscopy that shows the interaction between matter and radiated energy, is a specific form of photometry that measures light parameters in a particular range as a function of wavelength. The solution, meant for diffraction grating and geometry etendue of the display unit, is provided by a controller that tunes the grating pitch to accommodate any emitted/transmitted wavelength from a sample made of microplastics, their degraded forms and their potential retention, and ensures that all the diffracted wavelengths are concentrated on the required etendue. The purpose is not only to go below the current Infrared limit of 20μm microplastic size, or to suggest an Infrared spectrophotometry geometry capable of detecting micro- and nanoplastics in the range of (1nm–20μm) for integrated nano- and micro-scales, but also to transform most of the pivotal components to be directly wavelength-independent. The related controlled geometry solutions, from the controlled grating slit-width up to the controlled display unit etendue functions, are suggested for a wider generic range integration. The results from image-size characterization show that the following charge-coupled devices, nanopixel CCDs, and/or micropixel CCDs of less than 100nm are required on the display unit, justifying the Infrared micro- and nanoplastic-integrated spectrophotometry, and the investigation conducted with other electromagnetic spectrum ranges that suggests a possible universal spectrometer/photometer.

Optics doi: 10.3390/opt6030029

Authors: Shusen Chen Kangxian Guo

In this paper, the effects of localized surface plasmons on the nonlinear optical properties of a composite system are studied. The system operates by placing a metal nanoparticle next to a semi-parabolic quantum well under a terahertz laser field. Firstly, the energy expression of the semi-parabolic well in the terahertz laser field is derived via a Kramers–Henneberger transformation, and then the new energy levels and wave functions are solved by the finite difference method. Next, optical absorption coefficients and refraction index changes are derived according to quantum theory. Finally, the study shows that localized surface plasmons can cause a redshift in the peak position, while simultaneously weakening the peak value of optical absorption coefficients. The results confirm that the desired performance can be obtained by adjusting the radius of the particle, the distance between the particle and the quantum well, or the natural frequency of the quantum well.

Optics doi: 10.3390/opt6030028

Authors: Matías Mederos Guillermo Grazioli Elisa de León Cáceres Andrés García José Alejandro Rivera-Gonzaga Rim Bourgi Carlos Enrique Cuevas-Suárez

Purpose: The aim of this in vitro study was to evaluate the influence of different models of commercially available portable dental radiometers on the measurement of light irradiance emitted by light-emitting diode (LED) photocuring units. Materials and Methods: Eight LED photocuring units, all emitting light in a single-wavelength spectrum, were tested. Light irradiance (mW/cm2) was measured using six portable dental radiometers: four digital models (D1–D4) and two analog models (A1, A2). Digital model D1 was used as the reference (control). All measurements were conducted under standardized conditions, and each LED–radiometer combination was tested in triplicate. Data were analyzed using Sigma Plot 12.0 (Palo Alto, CA, USA) to verify the assumptions of normality and homogeneity of variances. A one-way analysis of variance (ANOVA) was used to assess the effect of the radiometer model on irradiance values, followed by Tukey’s post hoc test for multiple comparisons. The significance level was set at α < 0.05. Results: No statistically significant difference in irradiance was found between D1 (control) and D2. However, significantly lower values were recorded with A2, while D3, D4, and A1 produced significantly higher irradiance values compared to the control (p < 0.05). Conclusion: Irradiance measurements can vary significantly depending on the radiometer model used. Clinicians should be aware of this variability and are encouraged to regularly check the irradiance of the light-curing units used in daily practice, ensure their proper maintenance, and implement periodic monitoring to maintain effective clinical performance.

Optics doi: 10.3390/opt6020027

Authors: Silvânia A. Carvalho Stefano De Leo

This study experimentally demonstrates transverse symmetry breaking—a mechanism governing laser planar trapping—and distinguishes its unique role from related phenomena such as the lateral Goos–Hänchen shift and angular deviations. While the latter effects describe positional or angular beam displacements at interfaces, transverse symmetry breaking fundamentally alters the beam’s spatial symmetry, enabling unprecedented control over its intensity and phase profiles. Empirical results exhibit exceptional agreement with a recently proposed theoretical model, validating its predictive capability. Crucially, our findings highlight transverse symmetry breaking as a critical tool for tailoring beam profiles, advancing applications in optical trapping, structured light systems, and photonic device engineering, where symmetry manipulation unlocks new degrees of freedom in light–matter interactions.

Optics doi: 10.3390/opt6020026

Authors: Siqian Yang Xinqiang Wang Tingli Song Wei Xiong Song Ye Fangyuan Wang

When interferograms in space heterodyne spectrometers exhibit tilted or distorted fringes, significant errors may occur in the demodulated spectral information. To address this issue, we propose a method for interferogram correction based on automatic spectral analysis. Simulations on erroneous interferograms of monochromatic and polychromatic light demonstrate that this method effectively corrects fringe tilts and significantly improves spectral demodulation accuracy. The standard deviations between the corrected spectra and ideal spectra for monochromatic and polychromatic light are 0.016 and 0.019, respectively, compared to 0.104 and 0.127 for uncorrected spectra. Additionally, the method successfully corrects experimental interferograms of potassium and neon lamps, accurately demodulating characteristic peaks of potassium and neon emission lines. It also enables accurate displacement measurement in a Michelson interferometer experiment. This method, through automatic analysis and one-sided spectral correction, efficiently and accurately corrects erroneous interferograms and enhances spectral demodulation accuracy, showing broad application potential.

Optics doi: 10.3390/opt6020025

Authors: Piotr Kwiek

This paper presents the results of theoretical and experimental investigations of a Hong–Ou–Mandel interferometer in which an optical beam splitter is replaced by an ultrasonic wave. The ultrasonic wave acts as an acousto-optical beam splitter for light, which is based on the phenomenon of Bragg diffraction on an ultrasonic wave. The Doppler effect was considered in the theoretical considerations and confirmed experimentally. It has been shown theoretically and experimentally that the Doppler effect changes the frequency of two-photon states at the outputs of an acousto-optical beam splitter. The frequency of the two-photon state in the positive diffraction order is increased by the frequency of the ultrasonic wave, whereas in the negative diffraction order, it is reduced by the frequency of the ultrasonic wave. It should be emphasized that there are no states 1112 in the outputs (diffraction orders), which disappear as a result of Hong–Ou–Mandel interference; consequently, the probability of detecting coincidences of photons between the plus first and minus first diffraction orders is zero, as it occurs in the Hong–Ou–Mandel interferometer. The frequency difference between the two-photon states at the outputs of the acousto-optical beam splitter was confirmed by recording the two-photon beat phenomenon. The obtained results changed the current view that the Doppler effect caused by ultrasonic waves can be neglected in the interaction of correlated pairs of photons with ultrasonic waves.

Optics doi: 10.3390/opt6020024

Authors: Daniel A. Nolan

Higher-dimensional communications in optical fiber enables new possibilities, including increased transmission capacity and hyper-entangled state transfer. However, mode coupling between channels during transmission causes interference between channels and limits detection. In classical optical communications, MIMO (modes in modes out) is a means to deal with this issue; however, it is not possible to utilize this technology in quantum communications due to power limitations. Principal mode transmission is another means to deal with mode coupling and signal interference between channels. Conceptually, this can be used in quantum communications with some limitations. In this study, we numerically simulated this process using the time delay method and show how it can be implemented using two and four higher-dimensional quantum states, such as W or GHZ states. These numerical simulations are very illustrative of how the implementation proceeds.

Optics doi: 10.3390/opt6020023

Authors: Cristina M. Gómez-Sarabia Jorge Ojeda-Castañeda

We present a theoretical framework for designing optical masks, which are useful for implementing nonconventional Schlieren techniques. We revisit the use of effective transfer functions, which emphasize the role of symmetries in the design of coded masks. The proposed technique implements an optical autocorrelation of a mask, which is coded with the Barker sequences. For the same purpose, one can also use masks coded with the pseudorandom sequences. For the sake of completeness, we link our deterministic theoretical framework with a simple statistical model. The proposed technique may be useful for the automatic sensing of phase gradients.

Optics doi: 10.3390/opt6020022

Authors: Stefani Petrova Yoanna Kostova Martin Tsvetkov Angelina Stoyanova Hristina Hitkova Polya Marinovska Albena Bachvarova-Nedelcheva

In this study, TiO2-Fe2O3/polyvinylpyrrolidone (PVP) hybrids were prepared using the sol–gel method. The iron content in the synthesized samples was 10 and 20 wt%. The influence of PVP on the phase transformation, morphology and optical properties of the as-prepared hybrids was characterized by various physicochemical methods—XRD analysis, UV–Vis spectroscopy, IR spectroscopy and SEM. The obtained sol–gel powders were tested for photocatalytic activity against tetracycline hydrochloride in distilled water under ultraviolet and simulated solar light illumination. The obtained results were compared to commercial TiO2 P25 (Evonik). The investigated samples exhibited good photocatalytic efficiency for the degradation of tetracycline hydrochloride; however, better activity was demonstrated by the 90TiO2-10Fe2O3/PVP sample. The latter one displayed weak antibacterial action against E. coli ATCC 25922 in the presence of UVA light.

Optics doi: 10.3390/opt6020021

Authors: Daniel T. Cassidy

Changes in the refractive indices of a GaAs laser chip owing to bonding strain are investigated by two-dimensional (2D) and three-dimensional (3D) finite element method (FEM) simulations. The strain induced by die attach (i.e., the bonding strain) was estimated by fitting simulations to the measured degree of polarisation (DOP) of photoluminescence from the facet of the bonded chip. Changes in the refractive indices were estimated using the strains obtained from fits to DOP data. Differences between the 2D and 3D FEM estimations of the deformation and of the photo-elastic effect are noted. It is recommended that 2D FEM simulations be used as starting points for 3D FEM simulations. Elastic constants for GaAs in plane-of-the-facet coordinate systems for 2D (plane stress and plane strain) and 3D FEM simulations are given.

Optics doi: 10.3390/opt6020020

Authors: Nuren Z. Shuchi Tyler J. Adams Naz F. Tumpa Dustin Louisos Glenn D. Boreman Michael G. Walter Tino Hofmann

The increasing demand for optical technologies with dynamic spectral control has driven interest in chromogenic materials, particularly for applications in tunable infrared metasurfaces. Phase-change materials such as vanadium dioxide and germanium–antimony–tellurium, for instance, have been widely used in the infrared regime. However, their reliance on thermal and electrical tuning introduces challenges such as high power consumption, limited emissivity tuning, and slow modulation speeds. Photochromic materials may offer an alternative approach to dynamic infrared metasurfaces, potentially overcoming these limitations through rapid, light-induced changes in their optical properties. This manuscript explores the potential of thiazolothiazole-embedded polymers, known for their reversible photochromic transitions and strong infrared absorption changes, for use in tunable infrared metasurfaces. The material exhibits low absorption and a strong photochromic contrast in the spectral range from 1500 cm−1 to 1700 cm−1, making it suitable for dynamic infrared light control. This manuscript reports on infrared imaging experiments demonstrating the photochromic contrast in thiazolothiazole-embedded polymer, and thereby provides compelling evidence for its potential applications in dynamic infrared metasurfaces.

Optics doi: 10.3390/opt6020019

Authors: Young-Gu Ju

We designed lens systems for a smart-pixel-based optical convolutional neural network (SPOCNN) using optical software to analyze image spread and estimate alignment tolerance for various kernel sizes. The design, based on a three-element lens, was reoptimized to minimize spot size while meeting system constraints. Simulations included root mean square spot and encircled energy diagrams, showing that geometric aberration increases with the scale factor, while diffraction effect remains constant. Alignment tolerance was determined by combining geometric image size with image spread analysis. While the preliminary scaling analysis predicted a limit at a kernel array size of 66 × 66, simulations showed that a size of 61 × 61 maintains sufficient alignment tolerance, well above the critical threshold. The discrepancy is likely due to lower angular aberration in the simulated optical design. This study confirms that an array size of 61 × 61 is feasible for SPOCNN, validating the scaling analysis for predicting image spread trends caused by aberration and diffraction.

Optics doi: 10.3390/opt6020018

Authors: Francisco J. Ávila

The biomechanical and optical properties of the cornea are responsible for its functional response, structural integrity and refractive function. Corneal viscoelasticity is the cornea’s ability to absorb transient increases in intraocular pressure (IOP) and constitutes a biomarker of glaucoma. The use of silicone hydrogel soft contact lenses (SiH-SCLs) can affect both corneal viscoelasticity and IOP. However, the behavior of the pure elastic and viscous components remains hidden within viscoelastic properties, and their influence and relationship with IOP in the biomechanical changes observed with short-term SiH-SCL use remains unknown. This study investigates the effects of silicone hydrogel soft contact lenses (SiH-SCLs) on corneal elasticity and viscosity and their influence on IOP over different lens wear periods: 10 or 20 consecutive days. Ocular Response Analyzer (ORA) measurements were combined with a biomechanical Standard Linear Solid Model (SLSM) to differentiate and calculate the elastic and viscous components of the cornea. The results showed that after 10 days of lens wear, elasticity and viscosity increased, with a significant reduction in IOP. After 20 days, elasticity and viscosity decreased, with a further reduction in IOP, reflecting a time-dependent effect of SiH-SCLs on corneal biomechanics. The study indicates the potential protective role of corneal viscosity against changes in IOP, which may be used for glaucoma treatment.

Optics doi: 10.3390/opt6020017

Authors: Zhaoliang Liu Peizhen Qiu Yupei Zhang

Automatic focusing is a crucial research issue for achieving high-quality reconstructed images in digital holographic microscopy. This paper proposes an automatic focusing method that combines the discrete cosine transform (DCT) sparse dictionary with edge preservation index (EPI) criteria for off-axis digital holographic microscopy. Specifically, within a predefined search range, Fresnel transform is utilized to reconstruct the off-axis digital hologram, yielding reconstruction images at various reconstruction distances. Synchronously, the DCT sparse dictionary is employed to reduce speckle noise, and the EPI is calculated between the denoised image and original image. The value of EPI is used as an indicator for assessing the focal position. A single-peak focusing curve is obtained within the search range 10 mm, with a step size of 0.1 mm. Once the optimal focus position is determined, a focused and noise-reduced reconstructed image can be simultaneously achieved.

Optics doi: 10.3390/opt6020016

Authors: Huixin Yuan Liang Zhao Weimian Guan Yuqi Yang Junjie Zhang

The mechanisms of material removal and structural transformation under laser radiation differ significantly between single-crystal diamond (SCD) and nanocrystalline diamond (NCD). This study employs atomic simulations to investigate the material removal mechanisms and structural transformation behaviors of SCD and NCD when subjected to laser irradiation. We analyze the effects of temperature and stress changes induced by laser radiation on structural transformations, revealing the driving mechanism behind graphitization transitions. Specifically, the thermal–mechanical coupling effect induced by lasers leads to graphitization in SCD, while in NCD, due to the stress concentration effects at the grain boundaries, graphitization preferentially occurs at these boundaries. The material removal processes for both SCD and NCD are attributed to thermal stress concentrations in the regions where the laser interacts with the diamond surface. This investigation provides a theoretical foundation for a more profound understanding of the behavior of diamond materials during laser irradiation.

Optics doi: 10.3390/opt6020015

Authors: Yao Koffi Jocelyne M. Bosson Marius Ipo Gnetto Jeremie T. Zoueu

In this work, we explore an optimization idea for the complexity guidance of a phase retrieval solution for a single acquired hologram. This method associates free-space backpropagation with the fast iterative shrinkage-thresholding algorithm (FISTA), which incorporates an improvement in the total variation (TV) to guide the complexity of the phase retrieval solution from the complex diffracted field measurement. The developed procedure can provide excellent phase reconstruction using only a single acquired hologram.

Optics doi: 10.3390/opt6020014

Authors: Abderrahim El Hat Imane Chaki Rida Essajai Abdelmajid Fakhim Lamrani Boubker Fares Mohammed Regragui Aziz Dinia Mohammed Abd-Lefdil

YbxEryZnO thin films with a low concentration (x = 5%, y = 0, 1, 3%) were made on glass substrates using the spray pyrolysis method. The films were characterized through the use of specific techniques to investigate their structural, optical, and electrical properties. The XRD structural analysis of the films revealed that they are polycrystalline with a hexagonal wurtzite structure and a preferential orientation in the (002) direction. The optical characterization of the co-doped layers in the range of 200 to 800 nm revealed that co-doping had a significant impact on the values of transmission. A well-defined peak in the infrared domain centered around 980 nm was observed in photoluminescence measurements. This peak signifies the transition between the electronic levels 2F5/2 (ground state) and 2F7/2 (excited state), proving that photons are efficiently transferred between the ZnO matrix and the Yb3+ ion. All layers exhibited n-type conduction and an electrical resistivity decrease to 6.0 × 10−2 Ω cm according to Hall effect measurements at room temperature.

Optics doi: 10.3390/opt6020013

Authors: Yuyang Shi Yan Long Xuhui Zhang Li Wei Bo Dai Dawei Zhang

Microlenses are essential optics widely used in many fields. The microfluidics-assisted fabrication method provides a rapid, convenient way to manufacture microlenses. However, there is no mathematical model to describe the profile of the liquid surface. This paper provides a theoretical explanation and mathematical model for the formation of the liquid surface of different curvatures in the microholes of different shapes. A numerical model based on the finite–difference time–domain method verifies the mathematical model. Furthermore, the optical properties of the microlenses in different shapes are analyzed through the demolded microlenses. The proposed theoretical model provides an analytical way to study the properties of microlenses.

Optics doi: 10.3390/opt6020012

Authors: Shuo-Tsung Chen Ren-Jie Ye Ching-Fu Chen Keng-Yuan Chang Yu-Hung Huang Sheng-Jie Tseng Jun-Qi Liu

Light-emitting electronic devices are widely used at present, and this increases the risk of myopia. Thus, the early identification of myopia and the prevention of further exacerbation has become an essential issue in ophthalmology. Recently, choroidal imaging has been used to assist in the early identification and prevention of high myopia due to the fact that swept-source optical coherence tomography (SS-OCT) is an essential diagnostic tool in ophthalmology. This study presents a novel method to judge high myopia using the SS-OCT image dataset obtained from a university hospital. In order to relate the proposed method to the region of the SS-OCT image, the curvature analysis of an arbitrary segmented curve similar to the region of the SS-OCT is first illustrated by quadratic functions and matrix operations. Next, the curvature formula is derived and then applied to the choroidal curve obtained manually in each patient’s choroidal image. In particular, we applied the curvature analysis and its results to find the maximal curvature and average curvature of each patient’s choroidal curve. Finally, we used the maximal curvature and average curvature to evaluate high myopia. The accuracy of the proposed maximal curvature method and average curvature method in the experimental results to verify the proposed method.

Optics doi: 10.3390/opt6010011

Authors: Hangning Kou Jingliang Gu Jiang You Min Wan Zixun Ye Zhengjiao Xiang Xian Yue

Wavefront sensing is an essential technique in optical imaging, adaptive optics, and atmospheric turbulence correction. Traditional wavefront reconstruction methods, including the Gerchberg–Saxton (GS) algorithm and phase diversity (PD) techniques, are often limited by issues such as low inversion accuracy, slow convergence, and the presence of multiple possible solutions. Recent developments in deep learning have led to new methods, although conventional CNN-based models still face challenges in effectively capturing global context. To overcome these limitations, we propose a Transformer-based single-shot wavefront sensing method, which directly reconstructs wavefront aberrations from focal plane intensity images. Our model integrates a Normalization-based Attention Module (NAM) into the CoAtNet architecture, which strengthens feature extraction and leads to more accurate wavefront characterization. Experimental results in both simulated and real-world conditions indicate that our method achieves a 4.5% reduction in normalized wavefront error (NWE) compared to ResNet34, suggesting improved performance over conventional deep learning models. Additionally, by leveraging Walsh function modulation, our approach resolves the multiple-solution problem inherent in phase retrieval techniques. The proposed model achieves high accuracy, fast convergence, and simplicity in implementation, making it a promising solution for wavefront sensing applications.

Optics doi: 10.3390/opt6010010

Authors: Benjamin Haas Rose Mary Kristian Cvecek Clemens Roider Michael Schmidt Michael Döllinger Marion Semmler

Standard endoscopy of vocal folds is in general limited to two-dimensional imaging. Laser-based 3D imaging offers not only absolute measurements but also the possibility of assessing all three spatial directions. However, due to human inter-individuality, a fixed grid configuration (with fixed edge length and spot size) does not necessarily provide the best coverage and resolution. We present a liquid lens optical design for a diffractive spot array generator with dynamic adjustment capabilities for both array size and spot size. The tunable nature of the liquid lenses enables precise control over the spot array generated by a diffractive optical element (DOE). The first liquid lens controls the spot divergence in the observation plane, while the second liquid lens adjusts the zoom factor. The optical configuration provides a dynamic range of 1.8 with respect to array size, significantly enhancing adaptability in imaging across various applications.

Optics doi: 10.3390/opt6010009

Authors: James Bounds Eshtar Aluauee Alexandre Kolomenskii Hans Schuessler

We present an empirical model for the cross-section of low concentration acetone gas in the range of 1671.5–1675 nm that encompasses the absorption band of the methyl stretch overtone. This model is experimentally validated with cavity ring-down spectroscopy (CRDS) measurements performed with a calibration gas and its diluted mixtures with breath samples. Particular attention is paid to accurate wavelength measurements with an interferometric wavemeter. The theoretical framework for analysis of gas mixtures with several absorbing species is presented. We show that the proposed empirical model can be used to accurately determine the concentration of acetone vapor in human breath samples. The comparison of the acetone absorption cross-section with previous results is also presented.

Optics doi: 10.3390/opt6010008

Authors: Mikael Lindgren Victoria M. Bjelland Thor-Bernt Melø Callum McCracken Satoshi Seo Harue Nakashima

Triplet–triplet transfer photochemical reactions are essential in many biological, chemical, and photonic applications. Here, the Pd-octaethylporphyrin sensitizer along with triplet–triplet annihilator (TTA) active 9,10-diphenylantracenes (DPA) and the related substituted variants in low concentrations were examined. A full experimental approach is presented for finding the necessary rate parameters with statistical standard deviation parameters. This was achieved by solving the pertinent non-analytical kinetic differential equation and fitting it to the experimental time-resolved photoluminescence of both slow fluorescence and sensitizer phosphorescence. The efficiency of the triplet–triplet energy transfer rate was found to be around 90% in THF but only around 75% in toluene. This appears to follow from the shorter lifetime of the sensitizer triplet in toluene. Moreover, the TTA transfer rate was on average more than 40% in THF toluene whereas a considerably lower value around 20–30% was found for toluene. This originated in an order of magnitude higher solvent quenching rate using toluene, based on the analysis of the delayed fluorescence decay traces. These are also higher than the statistically expected 1/9 TTA efficiency but in accordance with recent results in the literature, that attributed these high values to an inverse intersystem crossing process. In addition, quantum chemical calculations were carried out to reveal the pertinent excited triplet molecular orbitals of the lowest triplet excited state for a series of substituted DPAs, in comparison with the singlet ground state. Conclusively, these states distribute mainly in an anthracene ring in all compounds being in the range 1.64–1.65 eV above the ground state. The TTA efficiency was found to vary depending on the DPA annihilator substitution scheme and found to be smaller in THF. This is likely because the molecular framework over which the T1 excited molecular orbitals distribute is less sensitive for a longer lifetime of the annihilator triplet state.