Photonics

This paper presents a systematic study of active polarization imaging in turbid water, focusing on the comparison between circular and linear polarization and the independent separation of forward and backward scattering contributions. Underwater imaging experiments with turbidity levels ranging from 0 to 30 NTU demonstrated that the circular polarization scheme has comprehensive imaging performance significantly superior to the linear polarization scheme, such as more uniform background, lower residual speckle, and higher structural fidelity, because circularly polarized light has the physical advantage of slower depolarization in multiple scattering, enabling it to maintain a high polarization contrast between target signals and background noise. The independent contributions of forward scattering and backward scattering are derived by blackbody substitution difference method (BSDM). Grayscale histogram analysis results reveal that the dominant factor limiting imaging quality in high-turbidity water is the convolution distortion caused by forward scattering, rather than backward scattering, and the convolution distortion cannot be eliminated through simple polarization difference. By experimentally comparing circular and linear polarization schemes and applying BSDM to separate forward and backward scattering, this work confirms the superiority of circular polarization scheme and proposes BSDM as a reliable and easily implementable analytical method for turbid water polarization imaging.

In space-based gravitational wave detection missions, the test mass (TM), which serves as the inertial reference, must maintain extremely low residual acceleration noise. During in-orbit operation, Galactic Cosmic Rays (GCRs) and Solar Energetic Particles (SEPs) cause charge accumulation and additional electrostatic forces. Therefore, ultraviolet (UV) light is used to illuminate the interior of the inertial sensor and regulate the TM charge through the ultraviolet photoelectric effect. However, surface contamination, temperature fluctuations, and material aging can degrade the surface Quantum Yield (QY) of the TM. In addition, transient intense charging disturbances induced by SEP events can rapidly alter the charge state of the TM, thereby degrading the long-term performance and operational stability of the Charge Management System (CMS). To address these issues, based on the nonlinear UV discharge model of the CMS, this study investigates the effects of QY degradation and external charging disturbances on the charge-transfer dynamics and control performance of the CMS and proposes an adaptive disturbance-rejection charge control method that integrates a Fuzzy self-tuning PID (FPID) controller with an Extended State Observer (ESO). Simulation results show that the proposed method enables effective charge regulation under different illumination schemes and prescribed QY degradation and external charging conditions and exhibits greater adaptability than conventional PID and fuzzy PID controllers. The findings of this study provide a theoretical reference for the design of charge control systems operating in complex space environments.

Metasurfaces provide a versatile platform for vivid structural coloration by tailoring the visible spectrum at the subwavelength scale. A variety of research has been dedicated to broadening the color gamut including hue and saturation. However, the color gamut is limited in a 2D CIE map, which cannot exhibit chiaroscuro information of colorful images due to the lack of brightness tuning. In this work, we propose a passive, reflective and analyzer-free metasurface platform performing arbitrary HSB tunability and exhibiting full color generation by simultaneous spectral and polarizing filtration. A single plasmonic metasurface consisting of aluminum nanorods is demonstrated to generate multiple colors, which are determined by the in-plane dimension of the nanorod. Based on Malus’s law, continuous brightness tuning of these individual colors can be achieved by adjusting the nanorod orientation under linearly polarized illumination, without requiring an additional polarization analyzer in the reflected-light path. Finally, we numerically reproduce the artwork “Portrait of a Musician”, which features photorealistic presentation and stereoscopic impression. Such an approach shows great potential to simply control the color in HSB space with high spatial resolution, which is significant in the applications of next-generation compact reflective displays.