Liquid crystal-based reflective polarization volume gratings (PVGs), also known as a linear Bragg–Berry phase optical element or a member of volume Bragg gratings (VBGs), is a functional planar structure with a patterned orientation of optical axis. Due to the strong polarization selectivity, nearly 100% diffraction efficiency, large diffraction angle, and simple fabrication process, PVGs have found potential applications in novel photonic devices and emerging near-eye displays. In this work, we start from the operation principles and liquid crystal configurations to discuss the optical properties, including diffraction efficiency, angular and spectral response, and polarization state of the diffracted light. Specifically, we emphasize promising applications of PVGs for near-eye displays and novel photonic devices. Through analyzing the functionalities of PVGs with simulations, PVG-based novel devices are proposed. We further develop polarization volume lenses (PVLs) with high diffraction efficiency, low f/#, and large diffraction angles. Previously reported planar lenses are of thin form factor but with on-axis imaging and large f/#. By patterning PVGs with parabolic phase, the obtained PVLs exhibit a small f/#, high diffraction efficiency, and large off-axis diffraction angle. The PVLs offer a new design for near-eye systems, especially for augmented reality (AR) displays. Based on PVLs, we propose a new multi-focal-plane AR system with a polarization multiplexing method to eliminate the vergence-accommodation conflict.
Volume Bragg gratings (VBGs) have many applications including filters, wavelength multiplexing devices, and seethrough displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer show the advantages of nearly 100% efficiency, large deflection angle and unique polarization selectivity. Previous studies of transmissive and reflective PVGs are based on a planar architecture. In this work, we introduce slanted configuration, i.e. slanted cholesteric liquid crystals. The optical properties of these two different-type PVGs are investigated and compared. Specifically, we emphasize on the diffraction efficiency and polarization state of the diffracted light. Through comparing the experimental results with simulations, the existence of slanted PVGs is validated. We further report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. Previously reported PVGs are of high diffraction efficiency but with fixed diffraction angles. By transferring PVGs onto a flexible and stretchable substrate, the obtained PVG films exhibit high diffraction efficiency, tunable periodicity, and excellent flexibility. The PVG films offer tunable diffraction angles and Bragg reflection bands by mechanical stretching. Stretch-release cycles test is also performed to ensure the mechanical robustness and reliability. This PVG film is especially useful for laser beam steering and augmented reality (AR) waveguide coupler.
This paper reviews a few novel designs of superconducting nanowire single-photon detectors (SNSPDs). For polarization-insensitive SNSPDs, we present a fractal-inspired design and a waveguide-integrated design; for lowtiming-jitter SNSPDs, we present SNSPDs integrated with current reservoirs.