Optics

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 (${\mathrm{P}}_{\mathrm{out}}$), average bit-error rate (BER), and ergodic capacity ($\overline{C}$) 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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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. Full article

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 $K^{\prime }$ 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. Full article

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. Full article

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. Full article

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. Full article

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 (θ_e) 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. Full article

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. Full article

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. Full article

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. Full article

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. Full article