Abstract We propose and numerically investigate a functionalized long-period fiber grating (LPFG) sensor for simultaneous temperature and humidity detection based on reconfigurable optical skyrmions. Around the 1550-nm operating band, variations in temperature and relative humidity are converted into changes in the LPFG resonance wavelength, modal coupling coefficient, and relative phase delay. These amplitudephase perturbations reshape the output Stokes-vector texture, enabling environmental information to be encoded into skyrmion features including the skyrmion radius R sk , texture rotation angle Θ sk , Stokes parameter s 3 contrast, and skyrmion number N sk . Over the simulated sensing range of 10-90 °C and 10-90%RH, the skyrmion radius varies from approximately 0.97 to 4.57 µm, while the texture rotation angle spans nearly 0-200°. The generated Stokes textures maintain a skyrmion number close to -1, confirming the topological stability of the reconfigurable polarization field. Inverse retrieval based on the multidimensional skyrmion-feature vector gives mean absolute errors of 12.16°C and 4.22%RH for temperature and humidity, respectively. Sensitivity and robustness analyses further show that the skyrmion readout remains distinguishable under readout-noise variation and an operating-wavelength detuning of ±0.5 nm. These results indicate that functionalized LPFGs can serve not only as spectral sensing elements but also as compact topological-light modulators, providing a promising route toward fiber-compatible multiparameter sensing using skyrmionencoded polarimetric fingerprints.
Abstract This paper proposes and experimentally validates a microwave photonic system for joint blind source separation (BSS) and angle of arrival (AOA) estimation of multi-source signals. By integrating digital signal processing algorithm, the system enables effective separation of multiple signals and accurate angle identification. In the experiment, two RF signals at 18 GHz and 21 GHz are directly generated by an arbitrary waveform generator. The two RF signals are applied to a dual-drive Mach–Zehnder modulator (DDMZM) together with a local oscillator signal. The modulator is biased at its minimum transmission point, and the desired difference‐frequency component is selected. The photodetector converts the optical signal output from DDMZM into a down-converted intermediate-frequency (IF) signal. The waveforms of the mixed IF signal are collected by an oscilloscope. In the data processing stage, the JADE algorithm is used to perform BSS on the IF mixed signal, then two IF signals are separated and recovered. Subsequently, the separated signals are input into the MUSIC algorithm. Through spatial spectrum construction and peak searching, precise estimation of the incident angle of each signal is achieved. Experimental results show that, under the emulated AOA conditions, the estimation error over the range of 0° to 90° is within ±1.4°.
Abstract This study introduces a mode-locked pulsed random fiber laser that utilizes a nonlinear optical loop mirror (NOLM) and a random phase-shift fiber Bragg grating. The features of saturable absorption and inverse saturable absorption in the NOLM are replicated in the experiment. By precisely adjusting the polarization controller (PC) in the NOLM, a mode-locked pulse output with a fundamental repetition frequency of 40.45 kHz can be achieved, and fourth-order harmonic mode-locked is achieved when the pump power is set at 200.1 mW. Using a pump power of 350 mW, alternating bright and dark optical pulses at repetition rates of 40.45 kHz and 80.91 kHz can be achieved by continuously adjusting the PC. The laser has a maximum power variation of less than 0.299 dB and a maximum wavelength variation of less than 0.04 nm, indicating stability. The proposed laser is promising for low-repetition-rate pulsed applications.
Abstract Aluminium gallium nitride (Al 0 . 3 Ga 0 . 7 N) and gallium nitride (GaN) based grating couplers are an efficient solution for optical output coupling in photonic integrated platforms. This work, for the first time, compares Al 0 . 3 Ga 0 . 7 N and GaN based waveguide with a dual fill factor grating structure, revealing the combined effects of etching depth, operating wavelength, and incident angle on achieving high optical outcoupling efficiency. A full-vector finite-element simulation is used to optimize the key structural parameters, including grating period, ridge widths, fill factors, incident angle, and etching depth, to maximize output coupling efficiency. The optimized dual fill factor design exhibits a pronounced wavelength-dependent, etching depth and incident-angle-dependent response and achieves a significantly higher coupling efficiency than a conventional uniform Al 0 . 3 Ga 0 . 7 N/GaN surface grating. These results demonstrate a compact and fabrication-compatible Al 0 . 3 Ga 0 . 7 N/GaN grating design, providing a viable pathway toward high-performance optical output coupling in fully integrated photonic and optoelectronic devices.
Abstract Non-uniform illumination and strong noise in dark regions severely degrade the phase demodulation accuracy of electronic speckle pattern interferometry (ESPI) fringe patterns. To address this issue, this paper proposes an improved variational Retinex model integrated with the shearlet transform. Gradient smoothness and sparsity constraints are imposed on the illumination and reflectance components to correct non-uniform illumination and enhance texture details. Subsequently, the reconstructed image is decomposed via the shearlet transform, whose multi-directional and multi-scale properties enable effective separation of speckle noise from fringe structures through hard thresholding. The proposed method is validated on both simulated and experimental ESPI fringe patterns and compared with three existing methods. Experimental results demonstrate that the proposed method exhibits unique advantages in speckle noise suppression and effectively recovers weak fringe structures submerged by noise.
Abstract Structural coloration offers a chemically stable and high-resolution alternative to pigment-based color generation by engineering light–matter interactions at the nanoscale. Although various nanophotonic platforms have been developed to achieve vivid and tunable structural colors, polarization-selective color control remains largely governed by in-plane geometric anisotropy, often resulting in coupled spectral responses between orthogonal polarization states. Here, we report a meta-mirror-assisted optical cavity architecture that enables distinctly decoupled polarization-dependent color behavior. Under x -polarized illumination, the reflected color exhibits strong sensitivity to the short-axis radius of the anisotropic meta-atoms, allowing for diverse and continuously tunable color generation. In contrast, under y -polarized illumination, the spectral response remains largely insensitive to variations in both lateral radii, resulting in a polarization-locked single-color output. Importantly, this polarization-decoupled behavior is preserved across variations in cavity thickness, highlighting the role of vertical cavity engineering in stabilizing the optical response. This mechanism establishes a robust strategy for polarization-selective structural coloration and offers new opportunities for secure optical encoding, information multiplexing, and multifunctional photonic devices.
Abstract To explore nonlinear harmonic generation enabled by symmetry-breaking bound states in the continuum (BICs), we numerically investigate an all-dielectric metasurface composed of silicon split-ring resonators (SRRs) supported on a SiO 2 substrate. By laterally shifting the inner segments of the split ring, the in-plane symmetry of the unit cell is continuously tuned, driving the evolution from a symmetry-protected BIC to a high-Q quasi-BIC resonance. The linear response is characterized by transmission spectra, quality factors, near-field distributions, and multipole decomposition, revealing that the representative symmetry-broken mode is dominated by the electric quadrupole contribution. Owing to the strong field localization supported by this mode, pronounced nonlinear enhancement is achieved. For a representative symmetry-broken configuration with α = 18.82% under a pump intensity of 1 MW cm −2 , the third-harmonic generation (THG) conversion efficiency reaches 1.32 × 10 − 4 at 1558.6 nm. The THG efficiency exhibits a quadratic dependence on pump intensity. In addition, both direct and cascaded fifth-harmonic generation are observed, with the cascaded contribution being nearly two orders of magnitude stronger than the direct one. These results show that symmetry engineering of dielectric SRR metasurfaces provides an effective route for tailoring BIC-related resonances and enhancing nonlinear frequency conversion.
Abstract The self-focused Hermite–Gaussian (HG) modes are shown to influence the excitation of electron plasma waves (EPWs) in a density ramped inhomogeneous plasma. The distinctive transverse electromagnetic mode structure of HG beams results in greater self-focusing and more effective electron trapping. Over shorter distances, higher-order modes, like (0,2) and (0,3), allow for higher electron energy gain, whereas lower-order modes favor sustained acceleration. By suppressing diffraction, the plasma density ramp helps to keep the beam confined and promotes prolonged EPW excitation. Relativistic nonlinearities also contribute to self-focusing, which leads to higher EPW amplitudes and better energy transfer to electrons. Comparative analysis demonstrates that HG beams have better energy transmission efficiency because of their asymmetric intensity profiles and efficient electron acceleration mediated by the longitudinal electric field of EPWs. The possibility of HG laser beams in enhancing inertial confinement fusion technologies and compact laser–plasma accelerator designs is highlighted by these findings.
Abstract We establish the direct connection between the Mathieu and parabolic families of nondiffracting solutions of the two-dimensional Helmholtz equation, thereby completing the set of overlap relations among the four fundamental separable Helmholtz families. By combining the known Mathieu-Bessel connection with the recently derived Bessel-parabolic overlaps, we obtain analytical expressions for the Mathieu-parabolic overlap functions in terms of Mathieu-function Fourier coefficients and generalized hypergeometric functions. These results yield explicit bidirectional expansion formulas between parity-definite Mathieu and parabolic beams, as well as between complex helical Mathieu beams and traveling parabolic beams. We further extend the construction to the finite-energy Helmholtz–Gauss case, deriving the corresponding relations between Mathieu–Gauss and parabolic-Gauss beams. Numerical reconstructions are presented to verify the convergence and accuracy of the derived expansions.
Abstract An object can become externally indistinguishable from its background without becoming uninformative. We examine this possibility in diffusive optical cloaking by separating uniform, simple local, and flat local residual nulls. At a simple local null, the nominal residual vanishes, yet the Fisher information remains finite when the first derivative with respect to the parameter of interest is still observable. We derive this result for both Gaussian measurements and photon counts, examine the loss of identifiability caused by uncertainty in the cloak, and replace a regularized information-to-residual ratio with a constrained design criterion. In a coated-cylinder model, the null retains 62.0% of the Fisher information available from the corresponding bare inclusion. An independent photon-diffusion calculation, including absorption, optical boundary conditions, and Poisson noise, exhibits the same behavior. These results provide a practical dark-field principle for diffuse optics: suppress the calibrated background response while preserving a measurable parameter-sensitive channel.
Abstract Pixel reliability assessment in optical phase unwrapping has relied predominantly on deterministic scalar metrics that cannot capture the multi-dimensional nature of phase quality in practical interferometric measurements. We present a multi-input fuzzy inference framework for optical phase unwrapping that introduces linguistic reasoning for pixel reliability assessment. Unlike conventional deterministic approaches that rely on scalar curvature measures, the proposed framework evaluates pixel trustworthiness using three fuzzy variables: phase smoothness, directional isotropy, and local consistency. This approach provides adaptive reliability assessment, a capability that fixed-threshold deterministic methods fundamentally cannot achieve, without the training data demands of deep learning alternatives, while maintaining exact 2 π periodicity through graph-based non-continuous propagation. Validation on simulated and experimental data demonstrates equivalent performance to conventional methods under noise-free conditions, with enhanced robustness in challenging scenarios including moderate noise (1.67× RMS improvement) and opposing gradient discontinuities (4.4× improvement). The framework maintains sub-second processing times while offering tunable parameters for application-specific optimization. This fuzzy reliability paradigm introduces adaptive decision-making capabilities to optical phase analysis, particularly beneficial for digital holography and interferometric applications where noise and measurement artifacts are prevalent.
Abstract This paper addresses the critical interaction between power imbalance in multi-channel lasers and pointing error within space multi-wavelength laser communication systems. We analyze a space communication system using a multi-channel reconstructed equivalent chirped (REC) laser as the transmitter. Results show that pointing errors and inter-channel power imbalance together attenuate the received signal, with power imbalance exerting a greater impact. Since the bit error rate (BER) depends on received power, this degradation severely increases BER, potentially interrupting communication in low-power channels. To address this, we integrate a semiconductor optical amplifier (SOA) for power balancing across REC laser channels. This reduces the power difference to below 0.5 dB, restoring low-power channels to normal BER performance. Our key finding indicates that, prior to mitigating pointing error, leveraging the spectral accuracy of REC lasers must be combined with active power balancing. This work establishes the SOA-enhanced REC array as a robust solution for reliable space communication.
Abstract Bound States in the Continuum (BICs) have attracted extensive attention in photonics owing to their ability to support ultrahigh quality factors (Q-factors). In this work, the modulation mechanisms of BICs in germanium-based all-dielectric metasurfaces with double-periodic and triple-periodic asymmetric configurations are systematically investigated. For the double-periodic structure, a BIC mode with a Q-factor exceeding 10⁷ can still be sustained at the Γ point despite pronounced structural asymmetry. The underlying mechanism is attributed to the strong coupling between the electric quadrupole (EQ) and magnetic dipole (MD) moments, rather than to the conventional symmetry-protected effect. By tuning the spacing between the air nanoholes, a controllable transition from a BIC to a quasi-BIC (QBIC) is achieved, where the Q-factor exhibits an inverse-square dependence on the asymmetry parameter. Meanwhile, multiple QBIC modes are observed to merge near the Γ point, resulting in a significant enhancement of the Q-factor. Furthermore, in the triple-periodic structure, the simultaneous excitation and independent manipulation of two distinct BIC modes are realized. This work relaxes the stringent symmetry constraints traditionally imposed on BICs and provides a new strategy for the design of high-Q photonic devices operating in the terahertz and infrared regimes.
Abstract Since quantitative phase imaging provides an attractive, promising route for wavefront detecting and sensing, it is becoming a supplement to classical intensity imaging. Among various quantitative phase imaging techniques, single-shot ones can retrieve dynamic and even transient phase distributions; therefore, single-shot quantitative phase imaging is a useful tool, especially in dynamic detecting applications. This tutorial summarizes the state-of-the-art progress on single-shot quantitative phase imaging. First, single-shot tactics employed in quantitative phase imaging techniques, mainly holography, ptychography, transport of intensity equation imaging, and differential phase contrast imaging, are summarized. Moreover, applications of these techniques, especially live cell imaging, flow cytometry, laser wavefront sensing, and transient field measurements, are concluded, and current challenges and prospects of single-shot quantitative phase imaging are further addressed. Moreover, numerical simulation codes for the classical algorithms used in these quantitative phase imaging techniques are also provided. This tutorial comprehensively covers all the aspects of single-shot quantitative phase imaging, including techniques, applications, and prospects; thus, it can be a reference for the future development and improvement of quantitative phase imaging.
Abstract Optical tweezers are a technique to directly transfer the momentum of light to particles to manipulate them at will. They are a promising tool for contactless position-orientation control of various micro- and nano-scale objects. However, despite their long history, they still lack controllability for irregularly shaped particles. This is due to the fact that applied motion with optical tweezers depends on the optical response; in other words, how the light goes through the particles. For particles with a simple shape (e.g. spheres and rods), since their optical responses are simple and known, the light pattern can be pre-designed so that they can be controlled at will. On the other hand, for irregularly shaped particles, since their optical responses are complex and not known, they cannot be controlled with pre-designed light patterns. In fact, such particles start rotating irregularly when illuminated with conventional optical tweezers, making position-orientation control almost impossible. To break this limitation, a new approach, dynamically changing the light patterns of optical tweezers, has recently emerged. In our laboratory, we take an approach of what we call real-time adaptive optical tweezers, which conduct both data acquisition and adaptation of light patterns based on the data of the objective particles in real time. In this tutorial, we describe the basic principles of why this real-time adaptive approach is effective, then describe an example of how to build a system with optical tweezers that can demonstrate such an approach in experiments. First, we explain the method of treating irregularly shaped particles with optical tweezers based on the theory of rigid body control in robotics, and discuss why a a real-time adaptive approach can be effective. Next, we describe the design of a system that can realize this real-time adaptive approach, featuring a PC-based system composed of a microscope camera and a galvo mirror system for light pattern generation.
This paper presents a comprehensive investigation of digital metasurfaces for enhancing light trapping in ultra-thin crystalline silicon solar cells. Silicon nano-pixels were arranged into diverse geometries-including plus, cross, spiral, square, and fractal patterns-to exploit Mie resonances for broadband absorption enhancement. Full-wave numerical simulations identified the snowflake fractal as the most effective configuration among the designs considered. The multi-scale snowflake metasurface exhibited strong light-trapping performance across the 300-1100 nm wavelength range, achieving a photocurrent density of 13.98 mA cm-2. Electrical simulations incorporating realistic doping and recombination mechanisms further validated the device performance, yielding a short-circuit current density of 10.5 mA cm-2-representing a 4.2-fold improvement over a conventional planar solar cell lacking a digital metasurface. These results highlight the strong potential of fractal metasurfaces as a pathway toward high-efficiency, material-efficient photovoltaic technologies, enabling substantial performance enhancement in ultra-thin silicon architectures.
Abstract From the burnished curvature of ancient mirrors to the atomic precision of modern metasurfaces, the art of shaping light has reflected the evolving mastery of optics itself. Over the past few decades, the Pancharatnam–Berry (geometric) phase has revolutionised this pursuit, enabling the phase, amplitude and polarisation of light to be sculpted through geometry rather than path length. Here, we revisit the physical foundation of geometric-phase beam shaping and explore how liquid-crystal and metasur technologies have transformed it into a versatile and practical platform. Finally, we highlight emerging directions—hybrid LC–MEMS devices and ultrafast electro-optic materials—that may overcome the long-standing trade-offs among speed, reconfigurability, and precision in optical beam shaping.
Abstract Photonics design has undergone a significant transformation, evolving from empirically guided trial-and-error methodologies to highly automated computational frameworks. This review traces the progression of design paradigms in photonics, beginning with traditional physics-based approaches grounded in Maxwell’s equations and advancing through the development of numerical simulation tools and inverse-design optimization techniques. Methods such as genetic algorithms, particle swarm optimization, gradient-based adjoint optimization, and topology optimization have enabled the systematic discovery of photonic structures tailored to specific performance objectives. More recently, artificial intelligence (AI), including machine learning, deep learning, and reinforcement learning, has emerged as a powerful paradigm for modeling, optimization, and device discovery in high-dimensional design spaces. These approaches offer new opportunities for accelerating design cycles, improving scalability, and uncovering non-intuitive geometries. This review critically examines the evolution of these methodologies, discusses their respective strengths and limitations, and outlines emerging directions that aim to integrate physical constraints, fabrication awareness, and system-level optimization into next-generation AI-driven photonic design frameworks.
We present a planar beamforming metasurface that, unlike conventional architectures, generates optical beams through in-plane excitation rather than out-of-plane illumination. This configuration simplifies fabrication, enhances scalability, and enables cost-effective integration of all transceiver components. The device achieves a +/- 30 degrees field of view with an 4 degrees beamwidth and no sidelobes, making it particularly suitable for compact free-space optical communication systems. Energy efficiency is achieved through the integration of non-volatile Sb2Se3 phase-change materials, which significantly reduce power consumption while satisfying the stringent size, weight, and power constraints of NewSpace missions. The metasurface employs only 18 meta-atoms with a maximum length of 2 & micro;m, minimizing the energy required for beam steering and enabling wide-angle control via partial crystallization. Numerical simulations predict ultra-low switching energy (2.18 nJ per event) and a total worst-case consumption of 0.35 W, orders of magnitude lower than conventional tuning mechanisms. Far-field analysis confirms stable, well-collimated beam propagation, supporting the feasibility of the proposed metasurface architecture for intersatellite optical links.
Abstract The present work simulates and evaluates a novel lead-free double perovskite absorber in combination with various transport layers to achieve a high-performance solar cell with optimized efficiency. The lead-free perovskite solar cells (PSCs) are configured with ITO as the transparent conducting oxide, Cs 2 PtI 6 as the absorber, PDINO, PCBM, IGZO, PFN-Br, C 60 as electron transport layers (ETLs), CuI, CuSCN, NiO, PEDOT:PSS, Spiro-OMeTAD, CZTSe, CFTS as hole transport layers (HTLs), and carbon as the back electrode. A total of 35 distinct PSC configurations were designed by combining different ETLs and HTLs, and their performance characteristics were thoroughly analyzed and evaluated. Among all possible combinations, CuSCN used as the HTL exhibited the best performance compared to the other HTLs. Accordingly, five superior device configurations incorporating CuSCN as the HTL, various ETLs, and Cs 2 PtI 6 as the absorber were thoroughly investigated. Their performance was analyzed under different input parameters, including the thickness and defect density of the perovskite absorber layer (PAL), ETL thickness, PAL bandgap, back-contact work function, series resistance, temperature, and shunt resistance. The best performance is achieved with the architecture ITO/PDINO/Cs 2 PtI 6 /CuSCN/C, delivering a V OC of 1.3 V, J SC of 32.17 mA cm −2 , FF of 73.71%, and a PCE of 30.95%.
