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This paper presents a comprehensive study on spectroscopic Mueller matrix polarimetric imaging of cholesteric liquid crystals (CLCs) and the impact of large diattenuation on the polar decomposition of Mueller matrices. The wavelength of the probing light is designed to cover the regions outside, at the edges of, and inside the photonic bandgap of CLCs, enabling systematic analysis of polarization properties under varying diattenuation levels. By applying the polar decomposition method to measured Mueller matrices, we find that the depolarization power extracted from the decomposition is significantly affected by the magnitude of diattenuation. When diattenuation is large, the extracted depolarization power deviates substantially from the theoretical behavior of an ideal CLC. Finite-difference time-domain (FDTD) simulations and theoretical analysis reveal that measurement errors in the Mueller matrix are amplified when the diattenuation is large, which leads to abnormal results in polar decomposition. Our results indicate that the polar decomposition method should be applied with caution when the diattenuation is large.

Based on phase-change materials (PCMs), the photonic crossbar array offers non-volatile reconfigurability and high integration density, enabling efficient large-scale parallel photonic matrix multiplication. However, its scalability is fundamentally limited by optical transmission loss, impeding the practical implementation of large-scale matrix multiplication. To overcome this limitation, this study proposes a weight mapping strategy that enables larger convolutional computation to be executed efficiently within a scale-limited photonic crossbar array. A high-quality 4 × 4 photonic crossbar array with 3-bit precision modulation has been fabricated. Applied to an edge detection task, the mapping strategy encoded four different 3 × 3 operators onto the 4 × 4 photonic crossbar array, achieving a 225% improvement in computational efficiency. Moreover, when integrated into a photonic convolutional neural network, the strategy delivered a 96.7% classification accuracy on the MNIST dataset, showing excellent agreement with the theoretical simulation result of 96.84%. Our work opens a path toward large-scale photonic matrix multiplication under hardware constraints, advancing the development of photonic computing.

Adiabatic frequency conversion enables fast and efficient tuning of laser light by coupling it into an optical resonator whose eigenfrequency is varied on a timescale shorter than its photon lifetime. In this regime, the optical frequency follows the cavity resonance, allowing frequency shifts of several hundred gigahertz within sub-microsecond time - independent of optical power and without phase-matching constraints. While a linear dependence of the cavity resonance on a control parameter (e.g., applied voltage) suggests that arbitrary temporal signals could be linearly transferred to optical frequency changes, we show that this assumption fails near mechanical resonances of the resonator. Using a millimeter-sized lithium niobate whispering gallery resonator with a pronounced mechanical mode at a center frequency of 10.5 MHz, we observe strong deviations from linearity even when higher harmonics of the control signal coincide with this resonance. The experimental results are in excellent agreement with theoretical predictions. They demonstrate that mechanical resonances impose intrinsic limits on the linearity of adiabatic frequency conversion and other frequency control schemes based on the variation of the eigenfrequency of an optical cavity.

A 1 at.% doped Yb:LaF₃ crystal was grown by the Bridgman method. The Raman, absorption, and fluorescence properties at room temperature were investigated systematically. The phonon energy, absorption cross-section, and emission cross-section of the Yb:LaF₃ crystal were 362 cm^-1, 0.35 × 10^-20 cm², and 0.55 × 10^-20 cm², respectively. The laser characteristics of Yb:LaF₃ crystals were demonstrated, including continuously tunable laser and passively mode-locked laser. Using a birefringent filter, the continuously tunable laser was obtained with a tunable range from 1011 nm to 1031 nm. An ultrashort pulse laser of 56 ps was achieved using a semiconductor saturable absorber mirror as a saturable absorber, resulting in a repetition rate of 90.10 MHz and a maximum average output power of 132 mW. The mode-locked laser corresponded to a pulse energy of 1.47 nJ and a peak power of 28.76 W. These results, underpinned by the crystal's advantageous properties such as an intrinsically broad emission bandwidth (52 nm), indicate that the Yb:LaF₃ crystal is a promising material for developing tunable and ultrafast pulse lasers in the near-infrared regime.

Metalenses are crucial for miniaturized and highly integrated optical systems, yet their inherent chromatic dispersion restricts broader application. Although recent designs have improved achromatic performance, attaining ultra-broadband achromaticity across the near- and short-wave infrared spectra remains challenging, especially when reconciling processing feasibility with structural stability. In response to this challenge, a semi-embedded unit cell is proposed in this paper, which can effectively alleviate the trade-off between performance and fabrication feasibility. By simulating and analyzing a metalens with an aperture of 62µm and a focal length of 65 µm, we demonstrate that the proposed structure achieves achromatic focusing across the 1000-2400 nm wavelength range, even under oblique illumination with incident angles up to 23° (equivalent to a 46°field of view). Within the operating wavelength band, the focal plane shift of the metalens is limited to a maximum of 4.5%. It exhibits an average absolute focusing efficiency of 42.66%, a full width at half maximum (FWHM) close to the diffraction limit, and an average relative focusing efficiency of 49.65%. Additionally, the proposed metalens exhibits high robustness to variations in material properties and geometric parameters, maintaining stable performance under a refractive index tolerance of ±0.04, an embedded region height tolerance of ±5%, an exposed region height tolerance of ±3.75%, and a side length tolerance of ±4%. The proposed semi-embedded structure offers a novel and reliable approach for developing ultra-broadband metalenses, with strong potential for highly integrated imaging and on-chip photonic applications.

Tactile perception plays a vital role in artificial finger pulp skin, especially in regions responsible for grasping and touching tasks, where precise sensing of deformation position and applied force is critical. Conventional demodulation methods often fail to fully leverage temporal correlations in data during the pressing process, limiting the accuracy of tactile demodulation. To address this, we propose a tactile sensing system based on quasi-distributed Fiber Bragg Gratings (FBGs) integrated into artificial finger pulp skin, along with a two-stage hybrid LSTM-Transformer neural network (TSH-LTNN) to jointly reconstruct pressing position and force. The network trains a temporal demodulation model by constructing possible data variations over three consecutive time steps, where the LSTM captures short-term continuity, the Transformer extracts long-range dependencies, and an adaptive fusion module integrates their complementary features. Experimental results show that the proposed model outperforms existing methods. In the 0-30 mm pressing range and 0-14.71 N force range, the mean absolute error (MAE) for position prediction is 0.2331 mm (R² = 0.9971), and for force prediction, it is 0.303 N (R² = 0.9829). Compared to the Random Forest model, the TSH-LTNN achieves a 2.34% improvement in position R² and a 66.06% reduction in MAE. For force prediction, it demonstrates a 2.48% improvement in R² and a 31.94% reduction in MAE. These results confirm that the proposed system offers precise, stable, and real-time pressure-state demodulation, with strong potential for high-precision haptic feedback applications.

This Ultraviolet luminescence (UVL) peak near 3.28 eV, as a common defect-induced luminescence phenomenon, has been a subject of extensive study as its origin is linked to the impurity incorporation during growth, which is crucial for material quality control in GaN-based optoelectronic and microelectronic devices. However, the UVL-related transition mechanism is controversial. GaN microdisks grown by Na-flux method with c-plane and (101¯1)-plane, can provide what we believe is a new perspective to reveal UVL-related mechanism. The low-temperature photoluminescence (PL) demonstrates the stronger UVL on the c-plane compared to the (101¯1)-plane. With increasing power and temperature, there is a significant blue shift of UVL in the c-plane. Therefore, the UVL-related mechanism may be the DAP transition. In unintentionally doped GaN, we suggest that the shallow acceptor associated with this DAP defect may be Mg-related defect. SIMS results show that c-plane has a high concentration of Mg impurities, which is consistent with the PL results that the c-plane has a significant UVL peak intensity. It is also consistent with the conclusion that the UVL-related shallow acceptor is Mg_Ga. This work reveals the origin of ultraviolet luminescence in Na-flux GaN, providing new insights into the optical properties of GaN crystals grown by the Na-flux method.

Underwater optical signal detection in severely degraded environments remains a challenging task. In this work, we propose what we believe to be a novel dynamic vision-based underwater optical signal detection system. The signal detection performance is evaluated in a degraded underwater environment, including partial occlusion and turbidity. The system utilizes dynamic vision sensors from an event camera array, multi-dimensional integral imaging, and deep learning networks to achieve signal detections. In the experiment, optical signals are transmitted using a modulated light-emitting diode. The optical signals, after propagating through the degraded underwater environment, are captured by the event camera array in the form of event sequences. The event sequences are preprocessed as multi-dimensional event videos. The videos are classified by the vision transformer and gated recurrent unit network (ViT-GRU). The proposed system is compared to other relevant state-of-the-art frame-based approaches in terms of the detection performance evaluated by the Matthew correlation coefficient and the number of error symbols. For the experiments we conducted, the proposed dynamic vision-based underwater optical signal detection system with multi-dimensional integral imaging and ViT-GRU network outperforms other frame-based counterparts in degraded underwater environments. To the best of our knowledge, this is the first report on dynamic vision-based underwater optical signal detection using multidimensional integral imaging.

Photonic crystals (PhCs) supporting bound states in the continuum (BIC) offer a promising platform for realizing electromagnetically induced transparency (EIT) analogs, enabling slow light and coherent control via modal interference. Their practical performance, however, is constrained by finite size effects and angle sensitivity. Here, we overcome this challenge by engineering a photonic flatband with a symmetry-protected BIC at the Γ point. Subsequently, a surface lattice mode (SLM) is precisely aligned with the flatband BIC via fine-tuning of structural parameters, forming a Flat-EIT response. Simulations show that at incidence angles of 4° and 10°, the Flat-EIT PhC exhibits group delays of 60 ps and 18.2 ps, respectively, which represent a significant enhancement in slow-light performance compared to previously reported non-flatband structures. Our results may facilitate the pathway towards the practical application of photonic devices.

Mode-locked fiber lasers provide an ideal platform for investigating nonlinear phenomena in dissipative optical systems. In this study, we experimentally demonstrate a linear mode-locked erbium-doped fiber laser incorporating a Sagnac loop mirror, which enables tunable spectral filtering by precise polarization control. Through gradually tuning the intracavity polarization state, the laser exhibits rich transitions from stable single-wavelength mode-locking state into a distinct period-doubling vector soliton and harmonic mode-locking soliton states. Among them, the pulse train undergoes doubling and harmonic behaviors of the fundmental repetition rate, accompanied by alternating pulse intensities induced by the intracavity polarization rotation. Real-time dispersive Fourier transform (DFT) technology provides real-time measurement of the spectral evolution during the transition, confirming that the vector soliton evolution and harmonic dynamics can be enabled by polarization-induced modulation. Furthermore, dual-wavelength period-doubling vector solitons are observed in the Sagnac-filter-based fiber laser for the first time, with both wavelength channels exhibiting synchronized vector soliton dynamics. These results establish a clear link between intracavity polarization perturbations and nonlinear vector soliton pathways, and further highlight dual-wavelength fiber lasers as versatile platforms for exploring complex dynamical behaviors in nonlinear photonics and advancing applications in dual-comb metrology and ultrafast signal processing.