Polarization singularities in momentum space, including bound states in the continuum (BICs) and circularly polarized states (CPSs), render great potential in singular and chiral optics. As far-field topological signatures of Bloch modes in photonic crystal slab, the dynamic evolution of the polarization singularities is observed by tuning the geometric parameters of the slab itself. Here, we combine the photonic crystal slab with a reflected multilayer substrate in which interference between the reflected optical field and the upward radiation of the Bloch mode brings us an extra degree of freedom to manipulate the polarization singularities without breaking the structural symmetry. As the polarization singularities evolve with the spacing between the photonic crystal slab and the substrate, the CPSs and extrinsic BICs are both generated from charge reversal of the at-Γ intrinsic BIC and then collide with the off-Γ intrinsic BICs. Importantly, all BICs are topologically protected from splitting throughout the whole dynamic process owing to the preservation of structural symmetry. Our findings suggest an alternative route to pursue ultrahigh-$Q$ CPS resonances and merge diverse BICs, which is potentially applicable in chiral optics, topological photonics, and beam modulation.

Bound states in the continuum (BICs) in photonic crystal slabs are characterized as vortex centers in far-field polarization and infinite quality (Q) factors, which can be dynamically manipulated in momentum space to construct the singularity configurations with functionalities such as merging BICs for further suppress scattering loss of nearby resonance. However, the vast majority of research focuses on two types of intrinsic BICs for simplicity, because these polarization singularities affect each other, and are even prone to annihilation. Here, we introduce the extrinsic (Fabry-Pérot) BICs and combine them with the intrinsic BICs to merge diverse BICs in momentum space. The extrinsic BICs can move independently of the intrinsic BICs, providing an unprecedented degree of freedom to reduce the complexity of constructing merging BIC configurations. Interestingly, an interaction of oppositely charged BICs that is collision beyond annihilation is revealed, which only exchanges the topological charge of BICs but not affect their existence. Following the proposed strategy, four-types-BICs merging and steerable three-types merging are achieved at the Γ and off-Γ points, further boosting the Q factor scaling rule up to Q∝k _x ⁻¹⁴ and Q∝k _x ⁻⁶ respectively. Our findings suggest a systematic route to arrange abundant BICs, may facilitate some applications including beam steering, optical trapping and enhancing the light-matter interactions.

The electro-optical imaging system works under focus conditions for clear imaging. However, under unexpected laser irradiation, the focused light with extremely high intensity can easily damage the imaging sensor, resulting in permanent degradation of its perceptual capabilities. With the escalating prevalence of compact high-performance lasers, safeguarding cameras from laser damage presents a formidable challenge. Here, we report an end-to-end method to construct the wavefront coding (WFC) imaging systems with simultaneous superior laser protection and imaging performance. In the optical coding part, we employ four types of phase mask parameterization methods: pixel-wise, concentric rings, linear combinations of Zernike bases, and odd-order polynomial bases, with parameters that are learnable. In the algorithm decoding part, a method combined of deconvolution module and residual-Unet is proposed to furthest restore the phase-mask-induced image blurring. The optical and algorithm parts are jointly optimized within the end-to-end framework to determine the performance boundary. The governing rule of the laser protection capability versus imaging quality is revealed by tuning the optimization loss function, and the system database is established for various working conditions. Numerical simulations and experimental validations both demonstrate that the proposed laser-protection WFC imaging system can reduce the peak single-pixel laser power by 99.4% while maintaining high-quality imaging with peak signal-to-noise ratio more than 22 dB. This work pioneers what we believe to be a new path for the design of laser protection imaging systems, with promising applications in security and autonomous driving.

Photoelectric imaging systems usually pursue a small-sized point spread function (PSF) to acquire high-quality images, but it also poses safety hazards for the camera in certain scenarios. For instance, ultrahigh intensity light focused on the sensor may cause irreversible damage when the camera is subjected to unexpected laser irradiation. Furthermore, clear images captured are vulnerable to privacy breaches during online transmission. In this paper, we report a concept of the OpSecureCam with protection capability versus both laser damage and privacy breaches via PSF engineering. The design recipe of the PSF with simultaneous enhancements in energy spread ratio and information flux is derived. An end-to-end framework is developed under theoretical guidance to jointly optimize the PSF and the decoding network, maximizing both protection capability and imaging quality of the OpSecureCam. A wavefront coding system is built to realize the concept, wherein the pupil phase distribution is obtained through a hybrid phase retrieval method based on Gerchberg-Saxton and stochastic gradient descent algorithms. Experiment results demonstrate that the OpSecureCam reduces the peak intensity of the jamming laser on its sensor by 99.73%, while the encoded images are robust against various blind deblurring methods. After being decoded by the matched network, the intricate structures of the image are restored with high quality for target identification or information extraction, including text, QR code, and human face. Our work offers a compact and efficient solution to enhance the adaptability of imaging systems, which holds potential for applications in autonomous driving and security monitoring.

To enhance the recovery capability of the polarization defogging algorithm for foggy scenes, a polarization image defogging algorithm founded upon the dark channel principle and multi-scale fusion is put forth. Initially, the atmospheric light is estimated based on the polarization information. This is followed by the estimation of atmospheric transmittance, which is achieved by combining the dark channel principle with the multi-scale image fusion technique. Finally, the atmospheric scattering model is constructed to obtain the defogged image. The experimental results demonstrate that the proposed algorithm exhibits excellent performance, achieving a high correlation coefficient of relevance and low CIEDE2000 color difference values in simulated fog environments of varying concentrations. The defogged image is improved by 59.68 times in terms of contrast, and the information entropy is increased by 11.38%. In a real outdoor fog scene, the defogged image generated by this algorithm showed a contrast improvement of 25.24 times and an information entropy enhancement of 8.32%. The presented algorithm defogs images more effectively and naturally than existing algorithms, enabling the recovery of detailed information from fog-obscured scenes.

Photoelectric imaging systems typically employ a focal plane detector structure, rendering them vulnerable to laser damage. Laser damage can severely impair or even completely deprive the information acquisition capability of photoelectric imaging systems. A laser damage protection method based on a microlens array light field imaging system is proposed to prevent photoelectric imaging systems from laser damage. The technique utilizes the light field modulation effect of the microlens array to homogenize the spot energy, thereby reducing the maximum single-pixel receiving power at the image sensor. The method's effectiveness has been verified through numerical simulations and experimental validation. First, the laser transmission theoretical model of light field imaging is proposed. Then an experimental setup is established, and measurements are conducted to capture the spot profiles and intensity distributions on the imaging plane across various defocus distances. Finally, the impact of the propagation distance on the maximum single-pixel receiving power and suppression ratio of the light field imaging system is experimentally measured. The simulation and experimental results indicate that, with the proposed method, the energy suppression ratio can easily reach two orders of magnitude, significantly reducing the probability of laser damage in photoelectric imaging systems. (c) 2023 Optica Publishing Group