Physical Sciences

Abstract: This review summarizes progress in the performance of nitride emitters across the full spectral range, with particular emphasis on the evolution of external quantum efficiency (EQE). Nitride emitters are primarily based on InGaN quantum wells, while AlGaN quantum wells are used for ultraviolet operation. Device performance is governed by the intrinsic properties of these structures, which also determine key physical challenges across different wavelength regions. Blue InGaN LEDs achieve the highest efficiencies (~60-80% EQE), while green devices are limited to ~20-35% due to the “green gap,” with further reduction (~5-20%) toward longer wavelengths. In the ultraviolet, AlGaN-based emitters exhibit lower performance due to material and structural challenges, although steady progress is being made. Special attention is given to mechanisms limiting EQE, including efficiency droop in the green-red region, and ongoing efforts to mitigate these effects. Finally, perspectives for future applications of nitride-based quantum structures in optoelectronics are outlined.

Abstract: Lithium niobate on insulator (LNOI) has emerged as a promising platform for compact, low-loss phase modulators. The extant LNOI studies evaluate device performance almost exclusively through the Pockels effect, treating piezoelectric-photoelastic strain and thermo-optic drift as decoupled channels. Crucially, both mechanisms directly perturb the phase bias of a fiber-optic gyroscope (FOG), rendering them indispensable in sensing-oriented design. This work establishes a unified multiphysics model of an X-cut TFLN ridge phase modulator that self-consistently couples the electro-optic, piezoelectric-photoelastic, thermo-optic, and pyroelectric channels. The contributions of the four mechanisms are quantitatively decomposed under realistic FOG operating conditions, and the slab thickness, ridge top width, and electrode gap are systematically optimized to balance modulation efficiency against environmental robustness. The co-optimization of the ridge geometry and electrode gap design maintains the EO overlap factor near 0.55 while reducing the half-wave voltage requirement. This results in a half-wave voltage length of V_πL = 1.65 V·cm at a 4.4 μm electrode gap. The optimal gap shifts only marginally to 4.2 μm at 85℃, with V_πL increasing modestly from 1.65 to 1.93 V·cm (push-pull single-pass) across 25~85 °C, attributable to the intrinsic γ33 change of LiNbO₃. Furthermore, a substrate temperature rise of 60 K under operating bias induces a mode field weighted thermal residual of approximately −3.1 × 10^-5 (equivalent to −6.3 × 10^-6 in spatial-average form). This thermal residual corresponds to approximately 27% of the Pockels modulation depth at an applied voltage of 5 V. The present study demonstrates that the DC-coupled operation of TFLN sensor-grade modulators is viable across the full FOG temperature range without active thermal compensation. The results of the study provide quantitative design guidelines for high-performance, environmentally stable TFLN phase modulators in compact FOG systems.

Abstract: The major challenge limiting the application of terahertz(THz) technology lies in the significant attenuation of THz waves loss of THz waves during free-space transmission arising from water vapor absorption and gas molecule scattering. Compared with free space propagation, low-loss and stable transmission of THz wave can be achieved through the waveguide. Waveguide transmission at low THz frequencies has attracted considerable attention, particularly at around 300 GHz (0.3 THz). Among the various types of THz waveguides, hollow waveguides offer a simple structure, ease of fabrication, low cost, and excellent transmission performance in the THz regime. Here, we design and fabricate a low-loss THz metal dielectric hollow waveguide based on polypropylene (PP) tubing, where an external silver film coated on the PP tube forms a leaky-type hollow waveguide structure. The linear transmission loss is measured to be 1.35 dB/m at 300 GHz. By optimizing this low-loss THz hollow waveguide, we achieve a far-field THz digital holographic (TDH) imaging recording configuration for the first time. To evaluate the imaging performance, different types of samples are measured. Experimental results for a plastic plate with aluminum strips validate a lateral resolution of ∼2.5 mm. The proposed method holds potential as a powerful tool for investigating spontaneous phenomena in the THz band.

Abstract: We studied the Stokes signals generated by the Raman photoexcitation of dissolved oxygen in water. When a water sample is pumped with intense nanosecond radiation, Stokes signals of different origins are generated. These signals form a characteristic nonlinear diffraction pattern, exhibiting a central spot and concentric rings whose radii depend on the Stokes wavelengths. Most of the Stokes signals correspond to the stretching vibrations of water molecules. However, we also observed a small contribution from dissolved oxygen molecules. This contribution can be separated from the others using appropriate spectroscopic filters and analyzed with a spectrometer. We report on Stokes components assigned to singlet oxygen excitation detected in the central spot, as well as in the diffraction pattern’s ring structure. The signal detected from the ring exhibits a single peak, while that from the ring itself shows a two-peak structure. The two observed peaks are interpreted as Stokes signals corresponding to Raman transitions to the two lowest vibrational sublevels of the singlet-oxygen electronic state. We also report exponential growth in the Stokes signal, in agreement with the standard stimulated Raman theoretical model.

Abstract: We present a fast time-domain simulator for optical cavities capable of reproducing non-linear dynamical regimes arising from ring-down effect during resonance crossings at high mirror velocities. The model is based on a recursive formulation of the intracavity electric field as a sum over round trips, preserving the cavity memory while maintaining high computational efficiency. The simulator is designed to achieve three main goals. First, the boundary conditions of the cavity can be modified at each simulation step, allowing arbitrary time-dependent variations of both mirror positions and input electric field. Second, the sampling frequency can be flexibly chosen by the user, however, it is internally adjusted before effectively executing the simulation to remain consistent with the cavity round-trip structure. Finally, high computational efficiency was obtained by avoiding the repeated evaluation of the full electric field history. The framework is validated through comparison with experimental data from the Virgo interferometer during a mechanical excitation experiment, showing good agreement in non-adiabatic regimes. Due to its efficiency and flexibility, the simulator provides a versatile tool for time-domain studies of optical resonators and future applications in real-time control and reinforcement-learning-based lock acquisition.

Abstract: We demonstrate an 80-MHz, 350-mW, 120-fs, 770-nm femtosecond laser, based on a nonlinear compressed 1540-nm femtosecond fiber laser. The home-built 1540-nm fiber laser delivering the 80-MHz, 2.69-W, 269-fs laser pulses, was realized by employing the spectral pre-modulation and pre-chirp management inside the Er/Yb co-doped fiber power amplifier. Subsequent nonlinear fiber pulse compression stage was utilized to further nonlinearly compress the pulse duration to 128 fs, based on the Gaussian assumption. Detailed numerical simulation was also implemented to investigate the optical dynamics of the nonlinear compression process. A 0.5-mm-thick fan-out periodically poled lithium niobate (PPLN) crystal was finally utilized to generate the frequency-doubled, 350-mW, 770-nm laser pulses with a 120-fs pulse duration, based on the Gaussian assumption.

Abstract: This study investigates the effect of co-doping with nanographene on the properties of FTO layers produced by spray pyrolysis. The results show that co-doping with nanographene enhances phase separation processes within the bulk of the FTO layer. The inhomogeneities formed during phase separation are depleted of fluorine compared to the film bulk. Co-doping with nanographene also significantly modifies the optical properties of the FTO layers. Specifically, it alters the position and intensity of the peaks in the reflection spectra, indicating a change in the nature of the absorbing centers. Furthermore, the optical bandgap of the FTO layers decreases with an increasing degree of nanographene doping. Finally, co-doping with nanographene reduces the resistance of the FTO layers, which can be attributed to an increase in charge carrier mobility.

Abstract: Miniaturization of photonic integrated circuits is a long-standing problem in optical engineering. Nowadays, the most promising material platform for integrated photonics are anisotropic van der Waals materials due to overcoming the light diffraction limit. Here, we numerically study v-groove channel waveguides formed in a 50-nm-thick slab of the in-plane hyperbolic in visible and near-infrared ranges van der Waals material MoOCl₂. At the telecom wavelength 1550 nm, a channel supports a guided mode with an effective index 1.0206 and a decay length of 13.7 µm. We also design a Mach–Zehnder–type interferometric layout with a maximum splitter angle of approximately 7° for demonstration of a possible practical application in a telecom range and in-plane angular channel modes propagation characteristics. We demonstrate that using MoOCl₂ instead of gold leads to a tenfold reduction in the linear dimensions of the photonic integrated circuit. Therefore, we envision that by combining the extraordinary material properties of MoOCl2 with the v-shaped geometry of waveguides, one can make the integration density of photonic devices close to electronics.

Abstract: The integration density of photonic integrated circuits is fundamentally limited by evanescent field overlap and subsequent inter-channel crosstalk. Layered transition metal dichalcogenides (TMDCs) bypass these confinement constraints through intrinsic optical birefringence and high refractive indices. Here, we report the near-infrared optical constants and waveguide dispersion of molybdenum diselenide (MoSe2). Ellipsometry performed on centimeter-scale crystals yields an in-plane refractive index of 4.1–4.7 over 1000–2000 nm, with an extinction coefficient close to the sensitivity limit of the fit away from strong excitonic resonances. To validate the anisotropic dielectric tensor at the device scale, scattering-type scanning near-field optical microscopy (s-SNOM) was utilized to map the propagation of transverse-magnetic modes in 235-nm-thick exfoliated flakes. Spatial Fourier analysis of the edge-scattered near-field interference yields effective mode indices that precisely match the modeled dispersion. Using the verified dielectric tensor, finite-element simulations demonstrate that single-mode MoSe2 waveguides optically outperform equivalent tungsten disulfide (WS2) benchmarks. The enhanced evanescent field suppression in the claddings of MoSe2 waveguide increases the coupling length by a factor of 3.5, reducing the required routing pitch and enabling a 12.5% direct increase in on-chip integration density. The results identify MoSe2 as a high-index anisotropic platform for compact waveguiding in the near-infrared.

Abstract: Polarimetric color cameras are a forefront technology that simultaneously capture polarimetric and color information by analyzing polarization states across different color channels, commonly red, green, and blue. In general, each of these color channels can carry different polarization information. Therefore, measuring the polarization Stokes vector at several discrete wavelengths simultaneously and with the highest possible resolution is of interest in multiple research areas. Nonetheless, this situation has not yet been investigated in specialized literature, where it is still commonly assumed that all color channels transport the same polarization information. In practice, polarimetric color cameras often come with the difficulty of color filter overlapping. For instance, the green filter partially transmits red and blue wavelengths, causing polarization-color crosstalk. In this work, we present a method to solve this problem. In addition, Fourier domain demosaicing techniques are applied to interpolate the data and reconstruct the images. The present study demonstrates how the proposed method leads to a successful recovery of chromatic and polarimetric information on both synthetic and real-world datasets. To test our approach, narrowband light beams at three wavelengths (470, 554, 630 nm), with different spatial polarization and degree of linear polarization distributions have been simulated and validated with experimental data. The results demonstrate the feasibility of the method for accurate three polarization channels measurements.

Abstract: Post-compression based on self-phase modulation (SPM) is widely used for femtosecond pulse shortening. However, the influence of the driving beam wavefront on different compression schemes remains unexplored. Using a Yb-doped fiber laser (230 fs, 200 kHz), we experimentally compare pulse compression in a hollow-core fiber (HCF) and a multi-pass cell (MPC). The HCF compresses pulses to 27 fs with an efficiency of approximately 55% and improves beam quality via modal filtering. The MPC achieves 34 fs pulses with an efficiency of approximately 90% and exhibits a quasi-waveguide mode-filtering effect, substantially enhancing output wavefront quality even when the input wavefront is poor. High-harmonic generation (HHG) experiments show that the HCF-driven source yields a higher photon flux (1.25 × 10¹¹ photons/s) compared to the MPC-driven source (4.95 × 10¹⁰ photons/s). Using a second Yb-doped fiber laser (223 fs, 100 kHz), a cascaded MPC–HCF scheme generates 7.5 fs pulses with an overall efficiency of approximately 70%, enabled by employing a larger-core HCF in the second compression stage. HHG experiments performed with the compressed pulses demonstrate that spatial phase evolution is a critical parameter in post-compression design for such applications.

Abstract: The purpose of this study was to evaluate the repeatability and inter-device agreement of higher order aberration (HOA) measurements in scleral lenses (SLs) obtained with two Hartmann-Shack (HS) metrology systems with substantially different spatial sampling resolution. Sixteen SLs (4 symmetric spherical, 4 spherical with toric periphery, 4 symmetric aspherical, 4 aspherical with toric periphery) were measured three times each using the SHSOphthalmic Cito (54×54 lenslet array) and SHSInspect Prio (157×157 lenslet array). Sphere (D) and Zernike coefficients from 3rd to 5th radial orders were extracted for three different aperture diameters (3.00 mm, 5.00 mm and 7.00 mm). Root-mean-square (RMS) values were calculated for each radial order and aperture. Within-device repeatability was assessed using coefficients of variation (CV%) and intraclass correlation coefficients (ICC) and inter-device agreement was evaluated using Bland-Altman analysis and (ICC). Both devices demonstrated excellent within-device repeatability for sphere, RMS4 and Total HOA RMS (ICC: 0.994–1.000, CV ≤4%). RMS3 and RMS5 showed moderate repeatability (ICC: 0.591–0.964), attributable to their small absolute magnitudes rather than instrument instability. Inter-device agreement was excellent at 5.00 mm and 7.00 mm (ICC: 0.950–1.000, mean bias <0.006 μm), with a significant difference only for RMS3 at 7.00 mm aperture (p = 0.034). At 3.00 mm, significant systematic bias was detected for RMS4 (bias = −0.00102 μm, p<0.001) and Total HOA RMS (bias = −0.00092 μm, p<0.001), with the Cito underestimating values relative to the Prio. FSE design did not significantly influence inter-device differences. HS spatial sampling density influences HOA measurement accuracy in SLs. The 8.45-fold difference in sampling points (24,649 vs. 2,916) resulted in clinically relevant inter-device discrepancies at 3.00 mm, whereas agreement was excellent at 5.00 and 7.00 mm. As specialty contact lens designs increasingly incorporate wavefront-guided (WFG) corrections, standardised high-resolution metrology protocols are essential to ensure accurate HOA characterisation, particularly at smaller apertures where reduced lenslet sampling density has been shown to compromise inter-device agreement.

Abstract: This paper presents a metrological framework for calibration, verification, and periodic re-verification of a metrology-oriented bulk-optics stochastic optical AND gate with polarization encoding and ratio-based readout. In the considered architecture, the logical values are represented by orthogonal polarization states, and the module output is evaluated through the corrected H- and V-channel intensities. The proposed methodology treats the module as a functional measurement object whose acceptability is determined by output correctness under specified operating conditions. The framework includes an explicit optical power-budget model, detector-channel calibration for dark offsets and gains, and a basic module-level verification procedure. Two complementary acceptance criteria are introduced: a functional decision margin based on a forbidden threshold band and a signal sufficiency margin based on the corrected total signal. Their uncertainty-aware forms are used for conformity assessment. A model case study demonstrates initial calibration, nominal verification, and periodic re-verification under drift scenarios. The results show that correct ratio behavior alone is insufficient for metrological acceptance and must be supported by adequate signal level. The proposed approach provides a compact routine verification procedure, while extended diagnostics and boundary analysis are treated as support layers rather than mandatory steps of each verification cycle.

Abstract: Ultrafast physical random bit generators (PRBGs) are essential components for modern applications in secure communication, quantum cryptography, and artificial intelligence. While optical chaos-based PRBGs offer high-speed capabilities, conventional systems often rely on discrete components that suffer from system complexity and environmental instability. This paper proposes and experimentally demonstrates a robust, integrated solution using a two-section mutual injection DFB laser. The device was fabricated using the reconstruction equivalent chirp (REC) technique, which provides precise control over grating phase variation while utilizing low-cost, high-volume fabrication methods.The laser sections, each measuring 450 m in length, were designed with a free-running wavelength difference of 0.3 nm to ensure a flat optical spectrum and enhanced chaotic dynamics. By optimizing the bias currents, we achieved a chaos RF bandwidth of 20.1 GHz. Notably, the resulting chaotic signal lacks time-delayed signatures, which simplifies the randomness extraction process. To generate random bits, the chaotic waveform was sampled by an 8-bit analog-to-digital converter at 100 Gb/s. Following post-processing through delay-subtracting and the extraction of the four least significant bits (4-LSBs), we realized a total physical random bit rate of 400 Gb/s. The randomness of the generated sequence was successfully verified using the NIST SP 800-22 statistical test suite. This approach offers a compact, energy-efficient, and high-performance integrated chaotic source suitable for secure communication and high-performance computation.

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.