Optics letters最新文献

Using luminescent technology for security verification holds promise for optical anti-counterfeiting. However, existing strategies often struggle in brightly lit environments due to ambient light interference. To address this issue, we introduce NaYF₄:Nd³⁺ as a flexible platform for ultraviolet (UV) luminescence in security applications. This phosphor exhibits readily detectable upconversion luminescence under intense white-light illumination, emitting at 354 nm-a wavelength beyond the typical indoor spectrum-for background-free signal detection. Driven by a two-photon excitation mechanism with an illumination threshold of 80 mW·cm^-2, UV imaging under daylight validates its reliability. This approach offers a robust, real-world solution for effective anti-counterfeiting.

1060nm vertical-cavity surface-emitting lasers (VCSELs) offer low dispersion and transmission loss for optical interconnects, but conventional oxide-confined designs face challenges in thermal accumulation and parasitic effects. We propose a high-speed 1060nm VCSEL featuring a hybrid confinement structure that integrates a buried tunnel junction (BTJ) with an oxide aperture, along with an optimized 8nm BTJ position offset. This hybrid architecture enhances current injection, reduces parasitic capacitance, and improves thermal management. The device achieves a -3dB modulation bandwidth of 37.2GHz, low differential resistance (~65 Ω), >50dB side-mode suppression ratio, and a >10K reduction in peak temperature via numerical modeling. These results demonstrate a promising approach for next-generation, energy-efficient VCSELs targeting high-speed, short-reach optical communication systems.

We experimentally demonstrated a high-performance multi-protocol continuous-variable quantum key distribution (CV-QKD) using one transceiver. The demonstrated multi-protocol CV-QKD system can simultaneously distribute 5 subcarriers with Gaussian, 4QAM, 16QAM, 64QAM, and 1024QAM modulation protocols based on orthogonal-frequency-division-multiplexing (OFDM) technology. Moreover, the modulation variance and probability distribution factor of 5 subcarriers are carefully optimized to maximize their respective SKRs. Besides, we innovatively design an effective OFDM-based DSP scheme and a precise dual-stage phase noise compensation scheme to realize a low excess noise for each subcarrier operating at a 2 GHz symbol rate. Thus, the multi-protocol CV-QKD achieves multiple SKRs of 48.79, 7.58, 23.67, 32.93, and 36.78 Mbps over a typical transmission distance of 50 km within a single transmission. Therefore, our work provides a practical solution to achieve highly efficient, interoperable, flexible, and secure QKDs for high-speed quantum secure communication.

Unilateral perfect absorption (UPA)-achievable through tailored system loss-holds broad application potential. It can be viewed as an extension of unidirectional reflectionlessness (UR), which is linked to non-Hermitian scattering exceptional points (EPs). However, the lack of a systematic design framework for UR-based UPA often results in confusion during the adjustment of coupling component parameters. Here, from a quasi-normal-mode perspective, we introduce a topology-supported design scheme that predicts and realizes UPA at an EP in continuum optical systems. By converting inter-resonator or inter-array coupling into interband coupling within energy bands, the asymmetry in mode radiation direction enables controlled asymmetric absorption at the EP. Maximum absorption asymmetry occurs under extreme radiation asymmetry, i.e., when unidirectional guided resonance (UGR) is supported. This framework provides what we believe to be new insight for asymmetric optical field manipulation and advanced photonic device design.

Quantum noise stream cipher (QNSC) is an encryption technique that leverages redundant information obscured by quantum noise to achieve an exponential increase in attack complexity, while continuous-variable quantum key distribution (CV-QKD) can provide such redundant information by generating secret keys. Notably, the optical architectures employed in these two schemes exhibit significant similarities. In this work, we employ polarization division multiplexing (PDM) to enhance channel capacity, enabling simultaneous transmission of signals under two different protocols over a single channel. To mitigate dynamic polarization state variations induced during signal propagation, we implement a polarization-interleaved subcarrier modulation (PISCM) scheme. We demonstrate that at a transmission distance of 20 km, the QKD secure key rate can be maintained at 4.93 Mbps, while the QNSC transmission rate reaches 100 Mbps. The expansion of the key rate from 4 Mbps to 400 Mbps using a linear feedback shift register (LFSR) enables synchronization with the 100 Mbps QNSC signal, realizing a promising pathway toward an integrated communication and encryption system.

Here, we demonstrate a multifunctional composite thin film based on metal-dielectric structure that simultaneously achieves visible transparency, infrared camouflage, and high-power laser protection. The inner side adopts an alternating stack of high- and low-refractive-index materials. Combined with a metal layer and outer dielectric layers, the structure utilizes optical interference to achieve a visible transmittance of 80.9%, while the structure achieves an emissivity as low as 5.9% primarily via metallic reflection. The dielectric multilayer composed of Ta₂O₅ and SiO₂ possesses a high laser damage threshold and continues to provide protection even after the outer layers are ablated. Under 1500 W/cm² laser irradiation for 120 s, the thin film exhibits exceptional damage resistance. This integrable solution offers significant potential for military optical windows, protection, and stealth system applications.

We proposed and demonstrated a high spatial resolution distributed twist sensor based on optical frequency-domain reflectometry (OFDR) in spun single-mode fiber (SMF). Due to the unique helical structure of the stress rods in spun SMF, external twist causes axial stress in the fiber, which leads to a frequency shift in the Rayleigh backscattering signal (RBS) spectrum. Distributed twist sensing with a spatial resolution of 1.56 cm is demonstrated. The measured frequency shift exhibits a cubic fitting relationship with the twist angle within the range of ±180°. Moreover, the proposed sensor is able to identify the twist direction, as twist in the opposite direction will cause a frequency shift in the opposite direction of the cross-correlation spectrum. The proposed sensor offers advantages such as high spatial resolution, distributed twist measurement, and twist direction identification, which is very promising in applications such as structural safety monitoring, medical intervention, and biomimetic design, etc.

Exceptional points (EPs) occur in non-Hermitian systems when two or more eigenstates coalesce, resulting in degenerate eigenvalues and distinctive physical phenomena that deviate significantly from conventional frameworks. Such singularities have attracted extensive research interest due to their potential in asymmetric mode switching, nonreciprocal transmission, and ultrasensitive sensing applications. Nonetheless, the practical implementation of EP-based sensors faces challenges arising from inherent fabrication imperfections, causing deviations from ideal EP conditions in rigid structural designs. Moreover, weak field coupling often limits the ability to detect exceedingly subtle perturbations. In this Letter, we introduce a reconfigurable EP sensor employing a high quality-factor (Q-factor) toroidal spoof localized surface plasmonic resonator (TSLSPR) with a suspended configuration, enhancing the reconfigurability of EP states across multipolar plasmonic modes. By strategically positioning two dielectric scatterers, our proposed EP sensor demonstrates significantly amplified frequency splitting compared to conventional diabolic point (DP) sensors, achieving an exceptionally low detection limit of 0.0001λ (diameter of 0.01 mm) and a notably high Q-factor of up to 423. This work presents an innovative approach for advancing high-sensitivity detection technologies and may stimulate further research into novel EP-based photonic devices.

A multispectral reconstruction method for flame color images based on K-means clustering and backpropagation neural networks (BPNN) is proposed to overcome the low spectral resolution of temperature measurement using color RGB three-band radiation images. A synchronized imaging system with an RGB camera and a 25-band multispectral camera was built to capture candle flame images. Image partitioning created a training set linking the three-band RGB and 25-band multi-spectral responses. Neural network training established a mapping between them. Spectral reconstruction of the candle flame images achieved an average relative error below 5%. The temperature inversion yielded an average error of 31.5 K, with a mean error of 1.79% in the error distribution, respectively, with test set R² values of 0.97-0.99, confirming high model accuracy. This work demonstrates the feasibility of merging the spatial advantages of RGB images with the spectral advantages of multispectral data, offering a new approach for dynamic flame temperature field monitoring.

Vector beams with spatially varying polarization, represented on the higher-order Poincaré sphere, are indispensable in optical communications, imaging, and quantum information science. However, nonlinear frequency conversion of vector beams remains challenging due to sensitivity and limited conversion efficiency. Here, we demonstrate a highly efficient, external-cavity-enhanced, polarization-independent frequency conversion based on difference-frequency generation for vector beams. Benefiting from cavity enhancement within the Sagnac configuration, we achieve external quantum efficiencies of 43.85% and 16.83% for vector beams with l=1 and l=2, respectively, demonstrating efficient continuous-wave frequency conversion. The preservation of the vectorial polarization structure in the converted beams is verified by spatial Stokes measurements. This work demonstrates an efficient approach for wavelength manipulation of vector beams and is potentially relevant to applications in structured-light control and nonlinear photonic interfaces.