Search Results (140)

Calcium fluoride (CaF₂) crystals are an ideal material for fabricating high-quality whispering-gallery-mode (WGM) optical resonators. However, their hard and brittle nature make it difficult to achieve low-damage, ultra-smooth surfaces through conventional cutting, which limits the optical performance of the resonators. To address this, laser-assisted diamond cutting technology is proposed in this study for low-damage and high-quality fabrication. Molecular dynamics simulations reveal the atomic-scale mechanism by which laser thermal effects reduce cutting forces and promote dislocation motion. Nano-scratch experiments further show that the critical depth of ductile–brittle transition (DBT) increases from 388 nm to 1070 nm, 2.76 times that of conventional cutting. Based on these results, ultra-precision turning of a high-quality hemispherical resonator with a Q factor of up to 1.3 × 10⁸ was achieved. This study provides an effective solution for low-damage and high-performance resonator fabrication from hard and brittle optical crystals. Full article

This study addresses the problem of whispering gallery mode (WGM) selection in spherical microresonators by means of their femtosecond micro-processing. The proposed method involves fabrication on the microsphere surface of defects playing the role of scattering elements for higher-order modes with low azimuthal mode indices. These two T-shaped trenches are created using femtosecond laser ablation, with a depth of 2 microns, gap of 30 microns between them, and each of length of 20 microns along the equatorial direction. A tapered fiber with a sub-micron waist diameter serves as the excitation element for WGMs. This method allows for spectral purification of the WGMs, reducing the number of resonances by 180 times, with a quality factor of $Q>{10}^5$ for the non-inverted spectrum in the form of resonance dips. Additionally, an inverted spectrum with narrow resonance peaks of about 35%, low background level and single mode regime with 3 dB side peak suppression has been simultaneously achieved in the taper transmission, for the first time to our knowledge. The latter was obtained by exciting the microsphere at the taper waist. These results hold promise for the development of narrowband filters, laser mode selectors, and optical sensors based on microresonators. Full article

This paper presents a systematic numerical investigation of a hybrid plasmonic–photonic Panda-ring antenna with an embedded gold grating, designed to enable efficient dual-mode radiation for optical and terahertz communication systems. The proposed structure integrates high-Q whispering-gallery mode (WGM) confinement in a multi-ring dielectric resonator with plasmonic out-coupling at the metal–dielectric interface, allowing controlled conversion of resonantly stored photonic energy into free-space radiation. The electromagnetic behavior is analyzed through a hierarchical structural evolution, progressing from a linear silicon waveguide to single-ring, add–drop, and Panda-ring resonator configurations. Gold is modeled using a dispersive Drude formulation with complex permittivity to accurately capture frequency-dependent plasmonic response at 1.55 µm. Power redistribution within the resonator system is described using coupled-mode theory, with coupling and loss parameters evaluated consistently from full-wave numerical simulations. Full-wave simulations using OptiFDTD and CST Studio Suite demonstrate that purely photonic resonators exhibit strong WGM confinement but negligible radiation, while plasmonic gratings alone suffer from low efficiency due to the absence of coherent photonic excitation. In contrast, the proposed hybrid Panda-ring antenna achieves stable and directive far-field radiation under WGM excitation, with a realized gain of approximately 8.05 dBi at 193.5 THz. The performance enhancement originates from synergistic hybrid SPP–WGM coupling, establishing a WGM-driven radiation mechanism suitable for Li-Fi and terahertz wireless applications. Full article

Monodisperse mesoporous hollow silica microcapsules present unique opportunities for advanced optical characterization due to their tunable nanostructure, high porosity and easy functionalization. A critical and challenging parameter in the optimization of these applications is the accurate determination of the effective refractive index, which governs light propagation and confinement within the nanostructured matrix of such mesoporous materials. In this study, individual mesoporous hollow silica microcapsules doped with Rhodamine B dye were analysed optically by exploiting whispering gallery mode (WGM) resonances, enabling non-destructive, single-particle refractometry with nanostructural sensitivity. Fourier Transform analysis of the fluorescence emission spectra revealed sharply defined, periodically spaced WGM peaks. For microcapsules with an 88 $\mu$m diameter, the measured intermodal spacing ($\Delta \lambda = 1.296$ nm) yielded an effective refractive index of 1.164. The measured value of the effective refractive index was cross-validated using Lorenz–Lorentz and Bruggeman effective medium models, both predicting porosity values (~63%) that closely match independent Brunauer–Emmett–Teller (BET) nitrogen adsorption measurements. The excellent agreement between optical and adsorption-based porosity demonstrates that WGM spectroscopy combined with Fourier analysis is a powerful, label-free, and non-invasive technique for correlating nanoscale porosity with macroscopic optical properties. This approach is widely applicable to single-particle analyses of nanostructured dielectric materials and opens new possibilities for in situ optical metrology in the development of advanced photonic, catalytic, and biomedical platforms. Full article

A reflective in-fiber liquid microsphere whispering gallery mode (WGM) resonator based on a Y-waveguide coupler is proposed and experimentally demonstrated. The sphere resonator is introduced inside a single-mode fiber (SMF) by using femtosecond laser micromachining and fusion splicing. A Y-waveguide coupler is fabricated with femtosecond laser direct writing, which is used to simultaneously excite and collect the WGM field through evanescent field coupling. High-index liquids are filled into the sphere through a laser-drilled channel to form a liquid microsphere where the WGM resonation takes place. The WGM resonator is sensitive to the refractive index (RI) of the filled liquids, and a RI sensitivity of 439 nm/RIU is achieved in an index range from 1.672 to 1.692. The liquid microsphere resonator is also sensitive to temperature, with a sensitivity of −307.1 pm/°C obtained. The microsphere resonator is small in size and robust, which has broad application prospects in the field of food and the chemical industry. Full article

Achieving controllable wavelength tuning in optofluidic whispering gallery mode microcavity lasers is crucial for high-throughput, multi-sample, multiplexed biochemical sensing and multifunctional integrated photonic devices. This paper develops a bidirectionally wavelength-tunable optofluidic fiber whispering gallery mode microcavity laser driven by Rhodamine 6G co-doped with different acceptor dyes. Experimentally, a thin-walled silica ring inside a hollow-core anti-resonant fiber served as the optical microcavity, with a fixed 2.5 mM Rhodamine 6G co-doped with other dyes as the gain medium. The results revealed that when co-doped with Rhodamine B or Cy3, the single-longitudinal-mode laser emission wavelength exhibited a red shift with increasing co-dopant concentration. Conversely, when co-doped with Cy5, the laser output wavelength showed a distinct blue shift. This unique bidirectional tuning characteristic originates from the different fluorescence resonance energy transfer efficiencies between the co-dopants and Rhodamine 6G, and their competitive modulation of the system’s effective gain spectrum. The study offers a novel and flexible strategy for achieving wide-range, controllable wavelength tuning on a single laser platform, with significant potential for applications in biochemical sensing and multifunctional integrated photonic devices. Full article

In recent years, there has been rapid advancement in single-molecule techniques, driven by their unparalleled precision in studying molecules whose sizes are beyond the diffraction limit. Among these techniques, optoplasmonic whispering gallery mode sensing has demonstrated great potential in label-free single-molecule characterization. It combines the principles of localized surface plasmon resonance (LSPR) and whispering gallery mode (WGM) sensing, offering exceptional sensing capabilities, even at the level of single ions. However, current optoplasmonic WGM sensing operates in a multiplexed channel, making it challenging to focus on individual binding sites of analyte molecules. In this article, we characterize different binding sites of DNA analyte molecules hybridizing to docking strands on the optoplasmonic WGM sensor, using the ratio of the resonance shift between sequential polar WGM modes. We identify specific docking sites that undergo transient interactions and eventually hybridize with the complementary analyte strands permanently. Full article

Dissipative sensing in a whispering-gallery-mode (WGM) microresonator entails monitoring changes in WGM throughput dip depth or linewidth due to analyte absorption. In our earlier work, we showed that dip depth sensitivity can be two orders of magnitude greater than linewidth sensitivity for sensing the broadband absorption of a dye in methanol. Here we experimentally demonstrate enhancement of absorption sensing of methane. Its narrowband absorption lines (a few GHz linewidth) necessitate strain tuning of the WGM of our hollow bottle resonator (HBR) to bring the WGM into resonance with the absorption line. Three asymmetric tapered fibers with different nonadiabaticities were designed to excite multiple fiber modes that couple into the WGM to interact with methane inside the HBR via the internal evanescent field. Measurements were carried out for both pure and trace (in 1 atm of air) methane at 1654 and 1651 nm. Enhancement factors as large as 141 were found; the experimental results agree with theoretical calculations and with the predictions of a limiting-case model. Effective absorption path lengths as large as 273 cm, more than ten thousand times the HBR diameter, were achieved for trace methane sensing, with detection limits estimated to be in the hundreds of ppm. Full article

Whispering-gallery-mode (WGM) microresonators are amongst the most promising optical sensors for detecting bio-chemical targets. A number of laser interrogation methods have been proposed and demonstrated over the last decade, based on scattering and absorption losses or resonance splitting and shift, harnessing the high-quality factor and ultra-small volume of WGMs. Actually, regardless of the sensitivity enhancement, their practical sensing operation may be hampered by the complexity of coupling devices as well as the signalprocessing required to extract the WGM response. Here, we use a silica microsphere immersed in an aqueous environment and efficiently excite optical WGMs with a free-space visible laser, thus collecting the relevant information from the transmitted and back-scattered light without any optical coupler, fiber, or waveguide. We show that a 640-nm diode laser, actively frequency-locked on resonance, provides real-time, fast sensing of dielectric nanoparticles approaching the surface with direct analog readout. Thanks to our illumination scheme, the sensor can be kept in water and operate for days without degradation or loss of sensitivity. Diverse noise contributions are carefully considered and quantified in our system, showing a minimum detectable particle size below 1 nm essentially limited by the residual laser microcavity jitter. Further analysis reveals that the inherent laserfrequency instability in the short, -mid-term operation regime sets an ultimate bound of 0.3 nm. Based on this work, we envisage the possibility to extend our method in view of developing new viable approaches for detection of nanoplastics in natural water without resorting to complex chemical laboratory methods. Full article

Integrated photonic biosensors are revolutionizing lab-on-a-chip technologies by providing highly sensitive, miniaturized, and label-free detection solutions for a wide range of biological and chemical targets. This review explores the foundational principles behind their operation, including the use of resonant photonic structures such as microring and whispering gallery mode resonators, as well as interferometric and photonic crystal-based designs. Special focus is given to the design strategies that optimize light–matter interaction, enhance sensitivity, and enable multiplexed detection. We detail state-of-the-art fabrication approaches compatible with complementary metal-oxide-semiconductor processes, including the use of silicon, silicon nitride, and hybrid material platforms, which facilitate scalable production and seamless integration with microfluidic systems. Recent advancements are highlighted, including the implementation of optofluidic photonic crystal cavities, cascaded microring arrays with subwavelength gratings, and on-chip detector arrays capable of parallel biosensing. These innovations have achieved exceptional performance, with detection limits reaching the parts-per-billion level and real-time operation across various applications such as clinical diagnostics, environmental surveillance, and food quality assessment. Although challenges persist in handling complex biological samples and achieving consistent large-scale fabrication, the emergence of novel materials, advanced nanofabrication methods, and artificial intelligence-driven data analysis is accelerating the development of next-generation photonic biosensing platforms. These technologies are poised to deliver powerful, accessible, and cost-effective diagnostic tools for practical deployment across diverse settings. Full article

We propose a low-frequency magnetic sensing method using a magnetically modulated microcavity resonant mode. Our magnetically sensitive unit with periodically changing magnetic poles is formed by combining an AC excitation coil with a microcavity. The microcavity vibrates at the frequency of the AC amplitude-modulated signal and changes its resonant mode when the sensing unit interacts with a low-frequency magnetic field. Signal processing is performed on the resonant spectrum to obtain low-frequency magnetic signals. The results of the experiment show that the measured sensitivity to a 0.5 Hz magnetic field is 12.49 V/mT, and a bias instability noise of 16.71 nT is achieved. We have extended the measurable frequency range of the whispering gallery mode microcavity magnetometer and presented a development in microcavity magnetic sensing and optical readout. Full article

A novel application of microresonators for refractometric sensing in aqueous media is presented. To carry out this approach, microspheres of different materials and sizes were fabricated and doped with Nd³⁺ ions. Under 532 nm excitation, the microspheres presented typical NIR Nd³⁺ emission bands with superimposed sharp peaks, related to the Whispering Gallery Modes (WGMs), due to the geometry of the microspheres. When the microspheres were submerged in water with increasing concentrations of glycerol, spectral shifts for the WGMs were observed as a function of the glycerol concentration. These spectral shifts were studied and calibrated for three different microspheres and validated with the theoretical shifts, obtained by solving the Helmholtz equations for the electromagnetic field, considering the geometry of the system, and also by calculating the extinction cross-section. WGM shifts strongly depend on the diameter of the microspheres and their refractive index (RI) difference compared with the external medium, and are greater for decreasing values of the diameter and lower values of RI difference. Experimental sensitivities ranging from 2.18 to 113.36 nm/RIU (refractive index unit) were obtained for different microspheres. Furthermore, reproducibility measurements were carried out, leading to a repeatability of 2.3 pm and a limit of detection of 5 × 10⁻⁴ RIU. The proposed sensors, taking advantage of confocal microscopy for excitation and detection, offer a robust, reliable, and contactless alternative for environmental water analysis. Full article

The optical Vernier effect has been widely studied due to its remarkable effect in improving the sensitivity and resolution of optical sensors. This effect relies on the overlapping envelope of two signals with slightly detuned frequencies. In the application of on-chip optical waveguide resonant cavities with whispering gallery modes, due to the on-chip space limitations, the length of the resonant cavity is restricted, resulting in an increased free spectral range. In the case of a small Vernier effect detuning, the required large Vernier envelope period often exceeds the available wavelength range of the detection system. To address this issue, we propose a novel on-chip waveguide structure to optimize the detection range of the cascaded Vernier effect. The proposed spiral resonant cavity extends the cavity length to 7.50 m within a limited area. The free spectral width (27.46 MHz) is comparable in size to the resonant linewidth (9.41 MHz), shrinking the envelope free spectral width to 371.29 MHz, which greatly facilitates the reading of the Vernier effect. Finally, by connecting two resonant cavities with similar cavity lengths in series and utilizing the Vernier effect, temperature sensing was verified. The results show that compared with a single resonant cavity, the sensitivity was improved by a factor of 14.19. This achievement provides a new direction for the development of wide-range and high-sensitivity Vernier sensing technologies. Full article

A new instrument for label-free measurements based on optical Low-Q Whispering Gallery Modes (WGMs) for various applications is used for a detailed study of the deposition and release of Layer-by-Layer polymer coatings. The two selected coating pairs interact either via hydrogen bonding or electrostatic interactions. Their assembly was followed by common Quartz Crystal Microbalance (QCM) technology and the Low-Q WGMs. In contrast to planar QCM sensor chips of 1 cm, the WGM sensors are fluorescent spherical beads with diameters of 10.2 µm, enabling the detection of analyte quantities in the femtogram range in tiny volumes. The beads, with a very smooth surface and high refractive index, act as resonators for circular light waves that can revolve up to 10,000 times within the bead. The WGM frequencies are highly sensitive to changes in particle diameter and the refractive index of the surrounding medium. Hence, the adsorption of molecules shifts the resonance frequency, which is detected by a robust instrument with a high-resolution spectrometer. The results demonstrate the high potential of the new photonic measurement and its advantages over QCM technology, such as cheap sensors (billions in one Eppendorf tube), simple pre-functionalization, much higher statistic safety by hundreds of sensors for one measurement, 5–10 times faster analysis, and that approx. 25, 000 fewer analyte molecules are needed for one sensor. In addition, the deposited molecule amount is not superposed by hydrated water as for QCM. A connection between sensors and instruments does not exist, enabling application in any transparent environment, like microfluidics, drop-on slides, Petri dishes, well plates, cell culture vasculature, etc. Full article

Optical microresonators supporting whispering-gallery modes (WGMs) have become a versatile platform for achieving electromagnetically induced transparency-like (EIT-like) phenomena. We theoretically and experimentally demonstrated the tunable coupled-mode induced transparency based on the surface nanoscale axial photonics (SNAP) microresonator. Single-EIT-like and double-EIT-like (DEIT-like) effects with one or more transparent windows are achieved due to dense mode families and tunable resonant frequencies. The experimental results can be well-fitted by the coupled mode theory. An automatically adjustable EIT-like effect is discovered by immersing the sensing region of the SNAP microresonator into an aqueous environment. The sharp lineshape and high slope of the transparent window allow us to achieve a liquid refractive index sensitivity of 2058.8 pm/RIU. Furthermore, we investigated a displacement sensing phenomenon by monitoring changes in the slope of the transparent window. We believe that the above results pave the way for multi-channel all-optical switching devices, multi-channel optical communications, and biochemical sensing processing. Full article