Abstract: Vertical-External-Cavity Surface-Emitting Lasers (VECSELs) have gained significant attention over the past two decades due to their versatility in a wide range of photonic applications. This review focuses on VECSEL configurations for dual-wavelength emission, highlighting their use in high-resolution spectroscopy, terahertz (THz) generation, and advanced optical communication. We explore recent developments in VECSEL designs, including systems utilizing birefringent crystals for polarization-based frequency separation and configurations with dual VECSEL chips or dual gain regions within a single cavity. These two-wavelength VECSELs enable diverse operation modes, including narrow-linewidth, pulsed, multimode, and frequency-converted emission, with high-brightness output, excellent beam quality, and tunable wavelengths. Additionally, the review discusses advancements in dual-frequency VECSELs, with applications in LIDAR systems for environmental monitoring, highly stable optical clocks, and fiber sensors. We examine improvements in cavity design, semiconductor structures, and power stabilization, which have enhanced frequency stability and spectral purity, making VECSELs suitable for precision metrology and sensing applications.

Abstract: The determination of the linear optical constants of solids is an important part of solid state optical characterization. Reflection spectroscopy and ellipsometry of surfaces or thin solid films represent established techniques to access those optical constants, however, they may suffer from an ambiguity of the obtained optical constants. We discuss methods for identifying the physically meaningful solution from the solution multiplicity, making use of a proper combination of independent measurements. Elaborating contours of constant reflectance (iso-reflectance curves) facilitates reliable identification of correct optical constants. A numerical criterion is further provided to select suitable combinations of measurements. The procedure is demonstrated in application to simulated spectra of a Nb2O5 film in the spectral region where the onset of the fundamental absorption edge is observed.

Abstract: The development of reproducible and stable plasmon-free substrates for surface-enhanced Raman scattering (SERS) is critical for practical applications in analytical chemistry. Transition metal dichalcogenides (TMDCs) have emerged as promising candidates due to their unique electronic properties, yet their performance is often constrained by the chemical inertness of their pristine basal planes. This work presents a systematic comparison of crystalline flakes and nanoparticles of tungsten diselenide (WSe2) and tungsten ditelluride (WTe2), prepared via liquid-phase ultrasonic exfoliation and non-equilibrium femtosecond pulsed laser ablation in liquid (PLAL), respectively. The results demonstrate that nanoparticle-based substrates consistently outperform their flake-based counterparts, achieving enhancement factors in the range of 104. The superior performance of the nanoparticles is attributed to the synthesis-induced defects and high-curvature regions in the nanoparticles shell which facilitates efficient, defect-mediated charge transfer between the substrate and the analyte. At the same time, the inner polycrystalline volume conserves the important characteristics of the bulk counterparts like excitons in semiconducting WSe2 and broadband absorption in semimetallic WTe2, which unblocks the tunable photothermal colloidal response. The study establishes morphology engineering through non-equilibrium synthesis as a powerful and generalizable strategy for designing high-performance, dual-function colloidal platforms, offering a pathway toward robust and reproducible analytical systems.

Abstract: A radial-polarization-based interferometric method is proposed for measuring object surface profiles. In the proposed approach, a radially polarized beam is generated by transmitting a linearly polarized beam through a zero-order vortex half-wave plate and is then introduced into a modified Twyman–Green interferometer, in which the test specimen is placed in one interferometric arm. By introducing a small variation in the wavelength illumination, two interferometric intensity patterns are recorded using a CMOS camera. The corresponding phase difference distribution is retrieved from the recorded intensities and subsequently used to reconstruct the surface profile of the specimen. The feasibility of the proposed method is experimentally validated by measuring a convex mirror, and the results show good agreement with theoretical predictions. Owing to its simple optical configuration, ease of alignment, high measurement accuracy, and rapid measurement capability, the proposed method demonstrates strong potential for practical surface profile measurement applications.

Abstract: This study presents an investigation of the morphology and electrical properties of ZnO as well as Ti-doped ZnO thin films, utilizing a fabricated digital spray pyrolysis device at 350 ^oC. Thin films of both ZnO and Ti-ZnO were prepared from extremely pure zinc acetate (Zn (CH₃COO)₂.2H₂O) as well as titanium dioxide (TiO₂) precursors. To change the concentration of the metallic components in the films, the precursors were prepared at 0.2 M using distilled water but was dissolved with the aid of hydrogen peroxide. ZnO with doped Ti films were prepared by combining the precursors in a mixture of Titanium Dioxide 0 to 10% of Zinc acetate. According to scanning electron microscope micrographs, the findings of both the undoped and doped films were seen to be evenly distributed across the substrates. The energy dispersive X-ray results indicated that Zn, O, and Ti were present in the films' elemental composition. The films I-V characteristics demonstrated an improvement of current as the doping increases. Thin films of ZnO doped with Ti produced in this investigation have morphological and I-V properties that make them suitable for use in photovoltaic solar panels.

Abstract: In the current paper, the nonlinear absorption characteristics and laser modulation performance of the ternary anisotropic semiconductor material ZrGeTe₄ were successfully explored. The recovery time of the ZrGeTe₄-PVA thin film was measured to be 5.74 ps by pump-probe technology. By employing ZrGeTe₄ as a saturable absorber, a passive mode-locked Yb-doped fiber laser was demonstrated for the first time. In the 1 µm mode-locked operation, the central wavelength is 1031.29 nm, the pulse repetition rate is 24.85 MHz, and the pulse width is 786.3 ps. In an Er-doped fiber laser operating at the wavelength of 1561.10 nm, the pulse width as short as 1.26 ps with a repetition rate of 4.38 MHz. The results show that ZrGeTe₄ has excellent broadband nonlinear optical characteristics.

Abstract: Hyperspectral imaging integrates spatial and spectral data, crucial for environmental and vision science applications. Existing spectroradiometers lack spatial resolution, and many hyperspectral systems are costly or unsuitable for fieldwork mimicking human vision. Here, we present the development of a portable, relatively low-cost hyperspectral camera based on a tunable filter and a commercial video camera, together with Python-based software capable of generating hyperspectral cubes from the acquired images, as a more accessible and adaptable alternative for specific applications. The proposed method includes assembling the acquisition system and processing raw images, which have been demosaiced, linearized, and labeled with their corresponding spectral band, to generate a hyperspectral cube. Our approach allows radiance retrieval through prior calibration or reflectance estimation using a white reference from a selected region of the hyperspectral cube. Additionally, it enables exporting the hyperspectral cube as a three-dimensional matrix. The developed software facilitates visualization and analysis of hyperspectral data. Validation against a spectroradiometer demonstrated reliable spectral radiance measurements under moderate to high light conditions. This adaptable approach enhances accessibility to hyperspectral imaging for research contexts with limited resources, supporting detailed visual environment characterization outside controlled laboratory settings.

Abstract: The thermal drift of microring resonators is one of the key obstacles hindering their practical applications. Employing polymers with negative thermo-optic coefficients to compensate for temperature-induced wavelength shifts represents a common solution. This study utilizes polymethyl methacrylate (PMMA) to compensate silicon nitride microring resonators, achieving thermal drift magnitudes below 2.0 pm/K within the temperature range of 15℃ to 70℃. Furthermore, nonlinear thermal drift characteristics were experimentally observed, and simulations revealed that these nonlinearities primarily originate from the temperature-dependent Young's modulus and Poisson's ratio of PMMA. This research provides design references for waveguide compensation using negative thermo-optic coefficient materials and proposes a conceptual framework for dual-function devices capable of both athermal operation and thermal tuning.

Abstract: Metal-enhanced fluorescence (MEF) has found widespread application in biomedical sensing and in vivo tissue imaging systems. To enhance MEF efficiency, it is necessary to optimize the interaction between metal nanoparticle plasmon and the fluorophore molecule. The size and shape of the nanoparticle, the nanoscale gap between the fluorescent molecule and the nanoparticle, and the excitation wavelength are critical parameters. In this study, we propose a model for a more complete and accurate description of the processes of molecular excitation and generation of the fluorescence spectral response, introducing new concept of effective properties for field enhancement factor, quantum yield, and fluorescence enhancement factor. The influence of the spectral properties of both the nanostructure plasmon and the fluorophore molecule on the optimal tuning of fluorescent complexes is studied. Particular attention is paid to the analysis of the spectral properties of plasmon resonance and calculations of the near-field intensity enhancement of the plasmonic nanostructure's excitation field. Numerical results for optimizing the MEF of fluorescent complexes based on TagRFP and gold (silver) nanorods composites are presented. The advantages of the proposed model for the optimal design of new nanomaterials with unique fluorescent properties are discussed.

Abstract: High-precision detection of hazardous gases with low refractive indices ranging from 1.000 to 1.100, specifically including methane, carbon monoxide, and sulfur dioxide, is critical for industrial safety, yet conventional sensors often suffer from limited sensitivity and severe thermal cross-sensitivity. This work presents a Magneto-Optical Differential Photonic Crystals Sensor (MO-DPCS) utilizing Indium Antimonide (InSb) to address these constraints. Employing the Multi-Objective Dragonfly Algorithm (MODA), the system was inversely optimized to maximize the magneto-optical polarization splitting while rigorously maintaining an ultra-high transmission efficiency. Crucially, an angular interrogation architecture operating under oblique incidence is established to maximize the magneto-optical non-reciprocity, where the detection is realized by fixing the terahertz source frequency and monitoring the precise angular displacements of the steep spectral edges. A differential detection technique was employed to utilize the non-reciprocal phase changes wherein Transverse Electric (TE) and Transverse Magnetic (TM) modes display contrasting kinematic characteristics in the presence of an external magnetic field. The findings indicate that with an adjusted magnetic field of 0.033 T, the MO-DPCS attains an exceptional differential sensitivity of 30.8 °/RIU, much above the 0.8 °/RIU seen in the unmagnetized condition. The differential approach efficiently eliminates common-mode thermal noise, minimizing temperature-induced drift to below 0.35° across a 1 K range. The suggested MO-DPCS offers a robust, self-referencing solution for stable and high-sensitivity gas sensing applications with a detection limit of 4.18 × 10-4 RIU.

Abstract: Flexible transparent conductive electrodes (TCEs) based on copper (Cu) meshes on polyethylene terephthalate (PET) substrates are constrained by critical interfacial weakness and inadequate mechanical durability, which hinder their widespread practical application. This study proposes a robust alloyed interface engineering strategy to address this fundamental challenge. Magnetron sputtering is employed to deposit Cu thin films on PET substrates with intermediate aluminum oxide (Al₂O₃) and nickel-chromium (NiCr) interfacial layers. Systematic comparative analyses reveal that the direct Cu/PET interface exhibits poor adhesion and mechanical fragility, while the incorporation of NiCr interlayers significantly enhances interfacial toughness. Through optimization, the NiCr layer forms a distinct alloyed interface with Cu via interdiffusion, fundamentally reinforcing the Cu/PET interface. Maskless photolithography enables precise patterning of Cu into micrometer-scale meshes, resulting in Cu Mesh/PET electrodes with excellent optoelectronic performance. The optimized electrodes achieve a sheet resistance of ~10.8 Ω/sq with an optical transmittance exceeding 87%, alongside remarkable mechanical robustness under repeated bending cycles. The synergistic toughening mechanism is clarified through interfacial microstructure analysis, which shows that the formation of a gradient alloyed zone effectively mitigates interfacial stress concentrations and suppresses crack propagation. This work provides a viable pathway for the development of next-generation durable flexible electronics.

Abstract: Dielectric engineered plasmonic nano-hole arrays (NHAs) provide an effective approach for controlling subwavelength light confinement. Here, we investigate wavelength compression in aluminum NHAs filled with three different dielectric materials such as Al₂O₃, MoO₃, and TiO₂ under illumination by a 1.5 µm lightwave. The hole radius varies from 300 nm to 500 nm to analyze the combined effects of geometry and dielectric environment on the plasmonic response. The NHAs filled with Al₂O₃ exhibit a pronounced and monotonic increase of the compressed wavelength with decreasing hole radius, indicating strong geometric tunability of the dominant plasmonic mode. Meanwhile, the structures filled with MoO₃ or TiO₂ show weak wavelength variations over the same radius range. Spatially resolved analysis at these nano-holes reveals nearly position-independent wavelength squeezing for Al₂O₃, whereas noticeable spatial variations appear for MoO₃ and TiO₂ at hole radii of 450 nm and 400 nm, respectively. The observed wavelength compression is attributed to hybrid plasmonic modes originating from the interplay between in-hole–like compressed cavity modes and localized surface plasmon polaritons. Our findings demonstrate how dielectric composition tunes wavelength compression in plasmonic NHAs, offering practical guidelines for designing the near-infrared plasmonic devices.

Abstract: In the AI era of big data explosion, similarity search—a core task in machine learning and data mining—requires high speed, energy efficiency, and scenario adaptability. Conventional electronic CAMs face RC delay bottlenecks, while existing OCAMs are limited by fixed bit-widths and limited distance metrics. Here, we demonstrate a variable bit-width all-optical CAM architecture employing phase-change material Sb₂Se₃ integrated with Mach-Zehnder Interferometers (MZIs). The proposed multi-segment memory unit (MSMU) compresses N-bit binary data into a single analog photonic unit, supporting direct data writing/loading without DACs and flexible trade-offs between precision, storage capacity, noise immunity, and energy, while enabling Hamming and non-linear (NL) distance calculations. A 6-element 3-bit OCAM fabricated on a SiN-SOI platform realizes reliable storage and retrieval. kNN simulations on iris, wine, and breast cancer datasets show that the 3-bit mode achieves accuracy comparable to Manhattan/Euclidean distances under high SNR, while the 1-bit mode offers robust noise immunity. Energy consumption is 364 fJ/bit (3-bit) and 890 fJ/bit (1-bit). This architecture provides a high-speed, energy-efficient, and flexible all-optical similarity search solution, promising wide applications in machine learning and data mining.

Abstract: Periodic supercell lattice structures with elements of random polydispersity disorder were created, to simulate the effect of randomization on photonic crystals, using finite-difference time domain (FDTD) methods. As a key exemplar system, a three-dimensional “inverse opal” structure of a face-centered cubic lattice with air spheres in a silicon dielectric was simulated, with sphere radii within supercells following a randomized Gaussian distribution, with characteristic standard deviation and mean. A corresponding ordered lattice with a bandgap with magnitude 3.5% of the normalized frequency range was used as a direct control, with sphere radius 0.34 times the lattice constant a. For a range of standard deviations, up to 5.9% of the 0.34a mean, a Monte-Carlo style approach was adopted, with photonic band properties analyzed over a large number of repeat simulations to ensure statistical significance. The corresponding Gaussian distribution in the resultant photonic bandgap magnitudes is broadened with increasing polydispersity such that an evolving fraction of simulations no longer exhibits a non-zero bandgap. A characteristic pseudo-transition occurs at a standard deviation of approximately 4.1% of the 0.34a mean, above where the frequency of simulations still returning a finite bandgap rapidly diminishes. Some isolated configurations, with a high degree of uniqueness, can exhibit enhanced bandgap properties (greater than the 3.5% benchmark) despite considerable polydisperse disordering; and we envisage that these findings point towards the use of engineered randomness in supercell systems to create desired photonic crystal properties and functionality, such as localization and optical bandgaps.

Abstract: Directly modulated VCSEL transmitters are widely deployed in short-reach optical interconnects, yet further scaling of per-lane symbol rates in AI/HPC datacenter fabrics demands modulation beyond the practical limits of direct current modulation. We demonstrate a laterally integrated VCSEL–electro-absorption modulator (EAM) transmitter enabled by resonance-detuned coupling on an oxide-confined half-VCSEL platform. A localized 20 nm surface etch produces >5 nm resonance detuning, confirmed by measured spectra and supported by transfer-matrix and mode-matching simulations, which indicate strong slow-light-assisted lateral coupling into the modulator. Experimentally, the measured spectra confirm an 5 nm resonance separation. Static characterization shows a coupling ratio of 63% extracted from near-field profiles and an extinction ratio of 4 dB (based on modulator-side power) under a −2 V modulator bias, with an apparent 1 mW absorption at a 6 mA VCSEL drive current. Dynamic measurements demonstrate a small-signal 3-dB bandwidth of approximately 23 GHz and clear NRZ eye openings at 25 Gb/s and 30 Gb/s. These results validate resonance-detuned lateral integration as a compact and manufacturable approach to VCSEL-based externally modulated transmitters for next-generation short-reach interconnects.

Abstract: GaN defect quantum emitters constitute a promising platform for single-photon generation in quantum information applications, yet many of their fundamental properties remain poorly understood. In this work, we perform experimental studies of the photoluminescence (PL) polarization of these emitters. Our findings indicate that the PL maintains a high degree of linear polarization across a broad temperature range (10-300 K), with only minor rotations of the polarization axis at intermediate temperatures. We discuss potential physical mechanisms responsible for this behavior and propose directions for future investigations.

Abstract: One of the main obstacles to testing quantum gravity is that genuinely Planckian effects are expected only at energies and curvatures far beyond current experimental reach. I point out that ultrafast, high-intensity lasers, in combination with fourth-generation x-ray light sources, already allow the laboratory production of non-inertial, effectively non-Minkowski space-time regions in which quantum fields experience enormous accelerations over micron-scale distances. By the equivalence principle, these configurations are locally indistinguishable from weak, highly curved gravitational backgrounds. I emphasize a concrete, experimentally motivated observable: the broadening of x-ray Thomson scattering from electrons accelerated in the focus of an extreme laser. This broadening depends on the acceleration and the local effective metric and hence can serve as a controlled probe of quantum mechanics and, ultimately, quantum-gravity-motivated modifications in non-Minkowski space-time.

Abstract: We simulate the propagation of a W states through an optical fiber in the presence of mode coupling. We illustrate the propagating quantum state graphically on a group of higher order Poincaré spheres. At the fiber output we show how to recover the input quantum state using the simulated quantum state information displayed on the multiple spheres. The geometry of these states is an SU(N) quantum geometry. Applications include higher dimensional quantum communications, quantum cryptography, and quantum networks, and longer-term quantum optical computing.

Abstract: Carbon dots (CDs) have enormous potential in optical sensing applications because of their remarkable physicochemical properties. Benefiting from high specific surface area, rich active sites, bright photoluminescence, high photostability, and biocompatibility, CDs have been widely used as functional layers in optical fiber sensors, resulting in notable improvements in sensitivity, response speed, and environmental stability. This review describes recent advances in CD-integrated optical fiber sensors, with a focus on CD synthesis techniques and their integration with optical fibers for the sensing of diverse analytes, such as heavy metal ions, biomarkers, and dyes. CD-integrated fiber sensors exhibit significantly enhanced detection performance in terms of sensitivity, selectivity, repeatability, response time, and recovery time, compared to their non-CD counterparts. Finally, current challenges and future perspectives are discussed. This review aims to provide valuable insights for the design and development of novel CD-integrated optical fiber platforms for sensing chemically and biologically relevant analytes.

Abstract: The effect of confinement of fluorophores (Rhodamine 6G) in nano-cavities of porous 3D sculptured coatings made by glancing angle deposition (GLAD) was investigated by fluorescence-lifetime imaging microscopy (FLIM). Shortening of fluorescence/photoluminescence lifetime by ~ 10% was observed from the dye-permeated (in liquid) structure; however, there was no rotational hindrance of dye molecules. When dried, a strong rotational hindrance 89% was observed for the orientation along the ordinary optical axis (fast-axis), and the hindrance was smaller than 57% for the extraordinary direction (fast axis). Light intensity distribution inside the nano-structure with a form-birefringence was numerically modelled using plane wave illumination and a dipole source. Nanoscale localisation of light intensity due to dipole nature I ~ 1/radius^6 and boundary conditions for E-field allows efficient energy deposition inside the region of lower refractive index (nanogaps).