Structured electromagnetic beams provide a unique degree of freedom for controlling light-matter interactions through their phase, polarization, and angular momentum. The influence of the total angular momentum projection, which combines the orbital and spin (helicity) contributions of the incident field, on multipolar scattering by nonspherical dielectric particles remains largely unexplored. We systematically investigate the scattering response of four canonical dielectric particle geometries illuminated by right-circularly polarized beams carrying total angular momentum projections m = 1 and m = 2 in the microwave regime. To enable rigorous full-wave simulations of structured-wave scattering by arbitrarily shaped particles, we employ an antenna-based structured-field excitation scheme together with exact Cartesian multipole decomposition. We show that the m = 1 beam simultaneously excites dipolar and quadrupolar resonances, whereas the m = 2 beam couples predominantly to quadrupolar resonances, yielding a quadrupole-dominated scattering response with the quadrupole purity exceeding 99% at the magnetic-quadrupole resonances of the sphere, cylinder, and cube and remaining above 90% at all quadrupole-dominated resonances. This mode-selective excitation remains robust against variations in particle geometry, aspect ratio, and alignment imperfections. At the same time, the quadrupole resonance frequencies exhibit pronounced sensitivity to geometric parameters, enabling precise spectral tuning. These findings establish a symmetry-driven mechanism for controlling multipolar scattering with structured electromagnetic fields and demonstrate that the proposed full-wave approach provides a versatile computational framework for investigating structured-wave interactions with complex dielectric resonators. The results highlight the potential of angular-momentum-controlled beams for mode-selective excitation and geometry-sensitive spectral tuning.

We report the Czochralski growth, spectroscopic characterization, and laser performance at 2.1 µm of a Ho:YScO3 mixed sesquioxide crystal with a doping concentration of 0.4 at.%. Spectroscopic evaluation was performed based on room-temperature absorption and fluorescence measurements. Intensity parameters, radiative lifetime of the [5]I7 manifold, and spontaneous emission rates were calculated based on Judd-Ofelt theory. For the [5]I7 → [5]I8 transition near 2.1 µm, the emission cross-section was derived to be 4.06 × 10[-21] cm[2], with an exceptionally broad full width at half maximum spectrum of 210.14 nm. In the laser experiment, the laser delivers a maximum continuous-wave output power of 12.1 W with a slope efficiency of 31%. Wavelength tuning over 85 nm (2074-2159 nm) is demonstrated using a Lyot filter. These results establish Ho:YScO3 crystal as a promising gain medium for high-power, broadly tunable, and potentially ultrafast 2.1 µm lasers.

The scattering characteristics of rough surfaces play a fundamental role in single-photon LiDAR, remote sensing, and optical target characterization. Conventional electromagnetic models describe the underlying optical scattering process, whereas photon-counting statistics and detector response must be modeled separately to describe measurements in the single-photon regime. In this study, a unified simulation model for rough-surface scattering and single-photon detection is developed based on the microfacet model. The model incorporates laser emission, photon propagation, surface scattering, and detector response within a single framework. Surface scattering is determined from the complex relative permittivity, root-mean-square height, and correlation length of the target, while photon absorption, specular reflection, diffuse reflection, and detector response are modeled using the Fresnel equations and photon-counting statistics. Simulations and experiments are performed for different photon transmittance levels, incident angles, material types, and surface roughness. The simulated and experimental scattered-photon distributions exhibit consistent trends and show good overall agreement for both metallic and dielectric targets. The proposed model provides an effective approach for analyzing rough-surface scattering and single-photon detection, offering theoretical support for single-photon LiDAR system design, optical target characterization, and rough-surface analysis.

Low-finesse Fabry-Pérot (F-P) cavities are attractive for high-precision micro-displacement measurement (MDM), but residual multiple reflections introduce periodic nonlinear error during quadrature demodulation. In this work, a reflective-sphere-based low-finesse F-P cavity and a model-based periodic nonlinear error compensation method are proposed. The reflective sphere is designed to provide a low effective return reflectivity and an approximately cosine-shaped interference response. Spectral measurements at working distances of 4-12 mm yield an average finesse of 0.492, close to the theoretical value of 0.5, and the measured free spectral range (FSR) values agree well with theoretical predictions. The compensation model parameters are identified from the sector-averaged radial distribution of the measured Lissajous trajectories. Under the parameter-identification condition of an initial cavity length of 2.0 cm, a nominal 4 µm peak-to-peak displacement, and a motion frequency of 3 Hz, the absolute displacement error is reduced from 150 to 30 nm after compensation. The same identified parameter set is directly applied without refitting to independent measurements and remains effective over the experimentally tested displacement-amplitude range of 2-6 µm and motion-frequency range of 3-9 Hz when the optical configuration is unchanged. A deliberate change in the initial cavity length requires parameter reidentification.

A controlled lateral offset is employed as a symmetry-breaking parameter to tailor multimode interference in an SMF-HCF-SMF interferometer for axial-strain and orientation-dependent curvature measurements. Numerical field-propagation and wavenumber-domain FFT analyses show that the lateral offset redistributes the relative spectral contributions of different optical-path-difference components. At the same time, the HCF length modifies accumulated phase and the effective-index-difference distribution. Under axial loading, selected spectral features exhibit linear blue shifts over a model-estimated HCF strain range of 0-4800 µε, with R[2] values of 0.974-0.994 and responsivities of -0.49 to -0.38 pm/µε. Opposite bending orientations produce reversed wavelength-shift polarities. The positive and negative orientations show monotonic nonlinear responses over 0.003-0.17 mm[-1] and 0.01-0.10 mm[-1], with range-averaged responsivities of 7.44 and -20.61 nm/(mm[-1]), respectively. A full 360° rotation further yields sinusoidal wavelength modulation, demonstrating azimuth-dependent spectral encoding. These results identify the collinear double offset as a controllable parameter for multimode spectral tailoring and orientation-dependent mechanical encoding in a compact fusion-spliced all-fiber platform.

We demonstrate a high-performance 660-nm vertical-cavity surface-emitting laser (VCSEL) employing a strain-compensated GaInP/AlGaInP active region and an asymmetric AlGaAs distributed Bragg reflector (DBR). By engineering a -10 nm room-temperature spectral detuning (λgain - λcavity), we compensate for the differential thermal redshift between the gain peak (0.26 nm/K) and the cavity mode (0.035 nm/K). This spectral realignment mitigates the severe Γ-to-X intervalley carrier leakage inherent to the AlGaInP system, sustaining high conversion efficiency at elevated temperatures. The fabricated device achieves a peak wall-plug efficiency (WPE) of 26.47% for 660-nm VCSELs and a continuous-wave output power of 19.11 mW at 300 K. The high efficiency is supported by reduced series resistance via graded DBR interfaces, which enhance thermionic-field emission and mitigate resistive heating. At 340 K, the VCSEL retains a WPE of 11.36% and maintains 40.87% of its room-temperature output power under 15 mA injection. These results demonstrate a robust design strategy for thermally stable red VCSELs in biophotonic and laser-display applications.

Efficient coupling of light into optical fibers is essential for a wide range of applications in telecommunications, sensing, imaging, and quantum photonics. Conventional fiber interfaces typically exhibit a limited angular acceptance interval, restricting their ability to efficiently collect and route light from off-axis sources. Here, we present a 3D-nanoprinted Metafiber that enables angle-selective coupling into a single-mode optical fiber while providing a broadband angular acceptance interval through a multiphase on-fiber grating lens. The device combines directional light coupling with suppression of undesired diffraction orders and is fabricated directly on spacer-enhanced fibers using high-resolution 3D nanoprinting. Optical characterization demonstrates efficient coupling over a substantially extended angular range while maintaining selective excitation of the fiber mode and strong zeroth-order suppression. These results establish a versatile Metafiber platform for directional light collection and spatial demultiplexing, offering new opportunities for fiber-based imaging, sensing, and integrated photonic systems.

Chirp control remains a key challenge for practical applications of water-window isolated attosecond pulses. In this numerical investigation, we propose thin liquid sheets as continuously tunable dispersive optical elements for chirp compensation of water-window attosecond pulses. The complex refractive indices of representative liquids are reconstructed by combining near-edge absorption data with tabulated X-ray optical constants and through the Kramers-Kronig relation. The resulting transmission and group-delay dispersion are then used to evaluate the compression of isolated attosecond pulses obtained from strong-field-approximation simulations. We show that the pre-edge dispersion of liquid water below the oxygen K-edge can compensate for the negative attochirp of long-trajectory harmonic emission in the 470-530 eV range, compressing the pulse from approximately 201 as to 67 as with a transmitted energy fraction of about 31.8%. Near the nitrogen K-edge, liquid nitrogen and liquid ammonia enable analogous compression in the 320-400 eV range, yielding pulse durations of approximately 54-56 as. These results indicate that free-standing liquid sheets can serve as flexible, self-refreshing, and thickness-tunable dispersive soft-X-ray optics for attosecond pulse compression.

Compact integrated terahertz (THz) systems require direct coupling between THz generation and wavefront manipulation. However, in most existing THz platforms, these two functions are still implemented separately. This study proposes and validates a monolithically integrated nonlinear-linear metasurface platform that combines Fano-resonance-enhanced THz generation with in situ broadband achromatic focusing. The nonlinear generation unit is based on an asymmetric plasmonic gold nanorod metasurface, where Fano-resonance-enhanced pump absorption and localized electromagnetic field confinement enable the generation of a broadband THz signal centered near 3 THz under infrared excitation. For broadband in situ wavefront manipulation, an all-dielectric silicon metasurface is integrated to achieve achromatic focusing across the 2.8-3.2 THz band. The integrated device maintains the focal position deviation within 10% of the designed focal length, with a focusing efficiency of approximately 40% over the operating band. This work provides a compact and scalable strategy for integrating nonlinear THz sources with dispersion-engineered wavefront manipulation, offering new opportunities for on-chip THz photonic systems, imaging, and spectroscopy.

We propose an interaction-free ghost imaging scheme based on a thermal light source. The scheme exploits the quantum Zeno effect to suppress photon absorption by the sample and thus light-induced damage. Crucially, this suppression requires only that the interfering paths be distinguishable in principle; no quantum detection or measurement results are needed, thereby removing the requirement for single-photon detectors. Moreover, our scheme is built on high-intensity thermal illumination. This is the key difference between this work and traditional studies of interaction-free imaging relying on weak or single-photon illumination. We demonstrate that replacing entangled photon pairs with a thermal source can improve image quality without sacrificing the essence of interaction-free measurement. Consequently, our scheme achieves quantum Zeno effect-based enhancement of the imaging performance without requiring specialized quantum hardware such as entangled photon sources or single-photon detectors. We further show active suppression of background noise via controllable photon loss. Our work offers a practical and cost-effective route to non-destructive, high-quality imaging for light-sensitive samples in fields such as life sciences.

Single-pixel imaging (SPI) based on Hadamard multiplexing enables efficient image acquisition with a reduced number of measurements, but its performance critically depends on the choice of subsampling strategy. In this work, we conduct a comprehensive evaluation of subsampling strategies for compressive Hadamard SPI, aiming to identify the method that selects the best-suited subset of Hadamard basis vectors. Natural images are known to contain most of their information in low spatial frequencies. Hence, the motivation for subsampling low-sequency patterns first, where sequency is the Hadamard analog of spatial frequency and corresponds to the number of sign changes in a Hadamard pattern, is intuitive but lacks theoretical grounding. This motivates the evaluation of multiple strategies which combine known subsampling schemes-deterministic, variable density sampling, and multilevel density sampling-with several reorderings of the Hadamard basis elements. These follow specific metrics defined on the Hadamard basis, including sequency, total variation, cake-cutting, and to our knowledge a novel data-driven variance-based ordering. In addition, spread spectrum and scrambled Hadamard approaches, relying on a randomization of the Hadamard basis, are considered. The performance of these strategies is assessed across different sampling ratios, Gaussian noise levels, and reconstruction methods, including pseudoinverse, ISTA, and a deep-learning-based approach (PinvNet), both on simulations and on an actual SPI setup. Results on a common, large dataset of images (BSDS500) indicate that deterministic selection of informative Hadamard patterns consistently outperforms probabilistic and randomized compressive sensing strategies. The proposed variance-based ordering achieves state-of-the-art performance for linear and sparsity-based reconstructions, while sequency ordering remains competitive for deep-learning-based reconstruction. These findings highlight the importance of selecting the most informative Hadamard patterns and open the way to data-driven approaches to assess their information content.

Exceptional points (EPs) in non-Hermitian systems provide a powerful platform for controlling electromagnetic waves. Here, we experimentally investigate a passive reflection-zero EP transition in a microwave spoof-surface-plasmon-polariton meta-strip supporting two coupled topological interface states. The mutual coupling between the interface states is systematically reduced by increasing their spatial separation. Temporal coupled-mode theory shows that the relevant transition is associated with the near-coalescence of the complex zeros of the left-reflection coefficient and their eigenvectors, rather than with the coalescence of the physical scattering poles. In the strong-coupling regime, two transmission maxima follow the two real reflection zeros. As the coupling decreases across the transition, the reflection zeros leave the real-frequency axis, and only one transmission maximum remains observable. The merging of the transmission maxima therefore provides a real-frequency signature of the reflection-zero transition but is not used alone as evidence of an EP. A localized dielectric perturbation produces a resolvable spectral splitting, illustrating the refractive-index sensing potential of this microwave topological platform.

Methane (CH4) is a potent greenhouse gas, and proper source attribution is essential for effective climate change mitigation. Traditional measurement techniques, such as singly substituted isotope ratio measurements (δ[13]C and δD) by IRMS or optical spectroscopy, often struggle to distinguish between different methane sources because their isotopic ratios can overlap. Doubly substituted isotope analysis, commonly referred to as clumped isotope analysis, offers improved differentiation. However, these isotopologues are challenging to detect due to their low abundance. In this study, we demonstrate the detection of a doubly substituted methane isotopologue ([13]CH3D) using cavity-ring-down spectroscopy (CRDS) at a wavelength of 4.5 µm, a region also used for the detection of [14]CO2. We further present a comprehensive data analysis routine for our CRDS measurements, including higher order mode filtering based on signal selection criteria, and relative and absolute frequency calibration. The approach is general and can be applied to other CRDS systems. Spectral lines recently assigned to [13]CH3D were addressed and characterized. The system achieves a precision of 19‰ at an averaging time of 50 minutes for [13]CH3D. Our results demonstrate that CRDS provides a sensitive and reliable approach for monitoring doubly substituted methane isotopologues on a shared platform with the capability for radiocarbon detection.

We present an oscillator-driven femtosecond transient microspectroscopy system based on supercontinuum generation in a photonic crystal fiber (PCF). The system operates at 80 MHz without pulse amplification, enabling high-sensitivity measurements with an RMS noise level below 10-4 at 500-800 nm spectral range while using low pulse energies suitable for microspectroscopy. Optimization of the PCF length (50 mm) suppresses dispersion-induced chirp, achieving a nearly uniform system response over 450-900 nm, which allows time constants below 100 fs to be resolved by deconvolution. The high-repetition-rate operation allows rapid spectral acquisition and efficient noise reduction. The developed system provides single-μm scale spatial resolution and supports both reflection and transmission configurations. The ultrafast carrier dynamics in multilayer and monolayer MoS2 are successfully resolved, including spatially dependent relaxation within a single monolayer flake. These results demonstrate a versatile platform for probing local ultrafast dynamics in low-dimensional materials.

Modulation instability (MI) is a well-known limit for distributed fiber sensors. However, self-phase modulation (SPM) is rarely addressed as a standalone constraint in distributed acoustic sensing (DAS). This study demonstrates, through modeling and experiments, that SPM-induced spectral broadening causes significant backscattered signal losses and increased phase noise. These effects are particularly critical when using high-power, short Gaussian pulses commonly used to extend the sensing range. We highlight the predominant role of the detection bandwidth in these excess losses, proving that SPM is a limiting factor independent of MI for high-resolution nonlinear ϕ-OTDR measurements.