Physical Sciences

Abstract: In femtosecond-laser processing of titania in water, light can induce reduction and oxidation simultane-ously. We follow this duality, in this perspective, from colloidal titania synthesis to hot-electron devices. Femtosecond ablation/fragmentation of an aqueous anatase suspension (515 nm, 230 fs, 5 µJ, fluence F ≈ 25.5 J cm⁻²/pulse at clamped intensity ∼1013 W cm⁻²) yields surface-reduced, Ti³⁺-rich bluish TiO_{2 – x}, while the same optical breakdown generates reactive oxygen species (ROS), among them H₂O₂ and HO• radicals, which compete by re-oxidising Ti³⁺. When the reduced titania is decorated with plasmonic nanoparticles (e.g., Au), an n-type plasmonic photo-electrode is realised: sp hot electrons are injected over the Schottky barrier, while the deep d-band supplies oxidising holes. Water oxidation proceeds in stages at potentials well above the formal 1.23 V via the two-electron peroxide route (∼1.77 V) or, for sufficiently energetic holes, via the one-electron HO• route (∼2.7 V). In a biased cell, H2 evolves on Pt through the adsorbed (H₂⁺)ad intermediate. The same Au/semiconductor physics on silicon enables sub-band-gap hot-electron photo-detection. Energy-level diagrams (flat-band and in-contact) and the sp- vs. d-band origin of the injected carriers are discussed.

Abstract: Optical microfibers are fabricated by pulling classical silica fibers until reaching diameters of a few micrometers or less. These devices are significantly exploited in many science and engineering fields, ranging from fundamental research to practical applications. Despite their many attractive advantages, a major technological challenge remains: heating caused by laser absorption from surface defects and contaminants. In the present study, we propose, for the first time to our knowledge, a novel method to measure the temperature evolution of laser self-heated microfibers in air at a wavelength of 1.48 µm. This method, simple and fast, enables us to investigate the influence of the diameters and lengths of the microfibers. We found that the temperature of the microfibers increases linearly with the power and measured a rise of 70°C for a 1 µm diameter and 20 mm length microfiber at a moderate power of 160 mW. A numerical model considering the microscale and the heat exchange with air is proposed and is adjusted with experimental data, providing values for the thermal transfer coefficient. By investigating power scaling, this work enables the prediction of temperature increases in self-heated microfibers in air, paving the way for new insights into the self-cleaning of microfiber-based devices and for optimized control of light propagation at high power levels.

Abstract: We demonstrate the integration of molecular beam epitaxy (MBE)-grown InP-based quantum dots (QDs) on gold thin films, achieving a fivefold enhancement of emission at telecom wavelengths compared with QDs grown on distributed Bragg reflectors (DBRs). Micro-photoluminescence (µ-PL) spectroscopy reveals a pronounced enhancement of the PL intensity, which is attributed to plasmon-assisted light–matter interaction at the metal–semiconductor interface. Reflectivity measurements reveal a characteristic dip near the QD emission wavelength, consistent with the excitation of surface plasmon resonances (SPRs), indicating favorable spectral overlap between the plasmonic modes and the QD emission. Power-dependent measurements reveal background-free exciton and biexciton emission from single QDs with resolution-limited linewidths. Polarization-dependent measurements demonstrate an ultra-small excitonic fine-structure splitting as low as ~2 μeV. Finally, a statistical analysis of multiple QDs confirms the reproducibility of the observed optical properties.

Abstract: Time-reversed Young (TRY) interferometry reconstructs interference from a fixed detector by reading out a programmable source-label distribution. We formulate this architecture as a source-coded Bayesian response sensor. For a perturbation parameter θ, the detected source-label histogram is a posterior distribution determined by a programmed source prior, an optical likelihood for a fixed-detector click, and an evidence factor equal to the click probability. The key point is not the Bayesian identity itself, but its physical implementation: in TRY the prior is imposed before propagation and can therefore reshape the response ensemble actually sampled by the detector. We show that the normalized posterior responds through a centered likelihood score, and that the detected-event Fisher information is the posterior variance of this score. This identifies posterior-weighted score contrast, rather than local response magnitude alone, as the relevant sensing resource. The framework separates posterior-shape information from evidence information, giving a resource-aware way to judge near-null response enhancement. It also yields practical design rules: a two-label source code converts a weak perturbation into a fixed-detector label imbalance, while the multiparameter score covariance provides a route to nuisance rejection and gives a minimal-label rank condition for sensing multiple perturbations. A passive double-slit implementation with weak one-slit phase and loss perturbations is proposed, requiring only fixed-detector source scans before and after calibrated perturbations. The results position TRY as a source-programmable Bayesian sensing architecture complementary to conventional detector-plane Young interferometry.

Abstract: Vortex optical states carrying orbital angular momentum provide an additional degree of freedom for fiber photonic systems [1-3], but their practical use requires efficient and reproducible excitation of guided modes with compatible transverse field structures. Ring-core and multi-ring-core fibers are natural platforms for such states because their annular guiding geometry can support modes with azimuthal phase variation [4-13]. However, a freely generated vortex beam does not automatically provide selective excitation of a prescribed guided mode. In this work, we develop a metasurface-assisted modal-matching framework for coupling vortex optical states into ring-core and multi-ring-core fiber architectures. The metasurface is treated not only as a vortex generator, but as a compact input interface that forms an amplitude–phase–polarization field profile optimized for a target vortex-compatible guided mode. A scalar modal-overlap model is used to evaluate target-mode coupling efficiency, modal purity, OAM-state purity, crosstalk, insertion loss, and tolerance sensitivity. The simulations show that a simple vortex field with the correct azimuthal order provides perfect OAM-state purity in the scalar basis, but only 0.616 target-mode coupling efficiency and 2.10 dB insertion loss. After radial modal matching, the coupling efficiency increases to 0.975 and the insertion loss decreases to 0.11 dB, while other-ring leakage is strongly suppressed. Practical non-idealities are also quantified: eight-level phase quantization still gives 0.950 coupling efficiency, whereas random phase errors with standard deviation reduce it to 0.856. The tolerance analysis identifies lateral displacement, beam-waist mismatch, ring-radius mismatch, phase noise, and polarization mismatch as the dominant limitations. The proposed framework provides a quantitative basis for designing structured-light input interfaces for selective excitation of vortex-compatible modes in annular fiber architectures.

Abstract: We numerically investigate high-efficiency third-harmonic generation (THG) via cascaded 2 χ⁽²⁾ processes in orientation-patterned GaAs (OP-GaAs) comprising two quasi-phase- 3 matched sections. The first section, with period Λ1 ≈ 212 μm, is phase-matched for second- 4 harmonic generation (ω → 2ω); the second, with period Λ2 ≈ 182 μm, for sum-frequency 5 generation (ω + 2ω → 3ω). Pumped by a continuous-wave CO₂ laser at λ = 10.61 μm, 6 the structure converts the fundamental to the third harmonic at λ3ω ≈ 3.54 μm, within 7 the atmospheric transmission window. By solving the coupled-wave equations with 8 temperature-dependent Sellmeier dispersion for GaAs, we optimize the number of domains 9 in each section and predict THG conversion efficiencies exceeding 90% at a pump intensity 10 of ∼ 100 MW/cm² in a crystal of total length ≈ 1.6 cm (N1 ≈ 66 domains in the SHG 11 section and N2 ≈ 100 in the SFG section). We also analyze the thermal tuning of the 12 quasi-phase-matching conditions over the range 22–300^◦C, and demonstrate robustness 13 against fabrication tolerances of ±10 domains and ±2% in domain length. Our results 14 establish two-section OP-GaAs as a practical, high-efficiency platform for mid-infrared 15 frequency tripling.

Abstract: One of the bottlenecks in realizing all-optical computing is the lack of on-chip all-optical logic devices that combine compact, low-loss, and highly robustness. Valley photonic crystals (VPCs) have become an important solution for realizing such devices, relying on the excellent transmission characteristics of topological valley states. However, existing structures still face issues such as limited design flexibility. In this paper, a high-performance topological all-optical logic device based on VPCs consisting of circular ring dielectric columns is designed and demonstrated. By introducing the inner radius as an independent design parameter, we construct a new type of VPC and systematically investigate its influence on the photonic band gap. Based on this, we design a beam splitter with high operational bandwidth and low insertion loss (<0.5 dB), and then realize fundamental OR and XOR logic gates, achieving extinction ratios of 18.9  dB for the OR gate and up to 44  dB for the XOR gate at an operating frequency of 193.5  THz. The platform also supports the NOT gate and, through cascading, can implement more logic functions such as AND gate.

Abstract: In this study, we present a thermal-aware design of a compact hybrid plasmonic grating (HPG) TE-pass polarizer on X-cut lithium-niobate-on-insulator (LNOI) for fiber-optic gyroscopes (FOGs). In a three-dimensional simulation, the optimization of the trapezoidal sidewall angle (θ = 78°) and the thickness of the Ag grating (13 nm) yield a polarization extinction ratio of 36.2 dB at 1550 nm (with a peak of 41.4 dB at 1548 nm) within a sub-10 μm grating length. This represents a ~3–8 dB improvement over prior LNOI HPG polarizers at the same footprint. A multiphysics thermo-optic analysis over the wide industrial FOG envelope (from −45 to +85°C) demonstrates that the operating-wavelength polarization extinction ratio remains within the range of 24.7–36.2 dB across the entire 130 K span (worst case 24.7 dB at −25°C), constrained solely by a modest 10 pm/°C Bragg detuning stemming from the pronounced (~5) thermo-optic anisotropy of LN. The insertion loss exhibits a negligible drift of merely 0.73 dB. A fabrication-tolerance study identified the Ag thickness as the predominant budgetary constraint (±1 nm tolerance, PER dropping ~10 dB at the resonance edge), while the ridge width and oxide buffer demonstrated comparatively greater flexibility. The device, therefore, fulfills the criteria for FOG-grade polarization suppression across the majority of the operational temperature range. The −25 °C point is established at the 25 dB threshold, thereby providing concrete design guidelines for ensuring environmentally stable on-chip polarization control on LNOI.

Abstract: The dynamics and synchronization properties of a network of three erbium-doped fiber lasers coupled via higher-order interactions (HOI) are investigated using simplicial complexes. Three coupling configurations are analyzed: first-order only, second-order only, and their combination. Several synchronization metrics, including the average synchronization error, similarity index, and correlation coefficient, are employed to characterize the effects of HOI. The results demonstrate that incorporating second-order coupling substantially enriches the system’s dynamical behavior, revealing a greater diversity of collective states than first-order coupling alone. While complete synchronization is not achieved within the bounded coupling strengths considered, both phase synchronization and anticipation synchronization emerge within distinct parameter intervals.

Abstract: Modern optoelectronic devices, such as light-emitting diodes and laser diodes, rely on nitride-based (GaN, AlN, InN) quantum structures, which underpin current technologies. These systems enable emission across a broad spectral range—from ultraviolet to infrared—with properties tunable via composition, strain, and quantum confinement. 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.