Reflective Polarization Volume Lens with Small f‐Number and Large Diffraction Angle

DOI: 10.1002/adom.202000170 then it could function as a converging or diverging lens, depending on the incident circular polarization.[3] Based on these attractive optical properties, many novel CLC devices have been developed and widely utilized in band pass filter,[4] laser beam steering,[5] optical combiners for augmented reality displays,[6] optical vortex generators,[7] beam shaper,[8] and many broadband devices.[9–11] Previously, transmissive planar lenses, such as LC lens and metalens, along the optical axis of a symmetric structure have been developed.[12–19] However, these lenses either have a small f/# but small size, or have a large aperture but large f/#. As the demand for compact device structure keeps increasing, such a transmissive planar lens becomes too bulky and the reflective planar lens is urgently needed to provide foldable optical structure. In this paper, we demonstrate a reflective polarization volume lens (rPVL) based on patterning slanted CLCs, exhibiting a large aperture and a small f/# (=0.825). In an rPVL, ideally the incident light is primarily reflected and focused onto a specific off-angle. To obtain these functions, such a device requires both asymmetric subwavelength alignment and lens phase for achieving reflective off-axis imaging and an ultralow f/#. In comparison with conventional transmissive planar lenses, our new rPVL with an f/# < 1 and 45° off-axis diffraction angle is a critical enabler for future compact optical systems.[20,21] PVL is a combination of a slanted polarization volume grating (PVG) and a lens. In the past studies, the LC director distribution in a CLC device was often assumed planar without slanted helical axis. Because the bottom photoalignment is planar, which produces cholesterics with nearly zero pretilt angle; this means that the helical axis is perpendicular to the substrate. Until recently, some experimental evidences and rigorous analyses prove that the actual LC director configuration in a PVG is not planar; instead, it possesses slanted structure.[22,23] Due to the trend of the lowest volume free energy, the helical axis of CLC bulk analyzed by the optimized Frank–Oseen model exhibits slanted rather than perpendicular to the substrate.[23] Therefore, the helical axis of rPVL is also slanted, which differs from conventional CLC devices. Figure 1a illustrates the schematic of the proposed rPVL. The substrate is treated to provide a rotation of LC optical axis in xy-plane, and the rotating angle changes continuously and Planar optics based on patterned cholesteric liquid crystals (CLCs) has attracted increasing attention owing to the self-organized helical structure and the ability to create arbitrary reflected wavefront through spatial orientation control. However, because of the subwavelength-orientation requirement, it is challenging for liquid crystal lens to achieve a low f-number (f/#) and large deflection angle simultaneously. Furthermore, with the increasing demand for compact size in novel optical systems, reflective lens has advantage over the transmissive one because it can fold the optical path. Here, a new off-axis reflective polarization volume lens (PVL) with f/# = 0.825, large aperture size, simple fabrication process, thin profile, circular polarization selectivity, and large diffraction angle is proposed. In contrast to traditional vertical spiral structure, PVL is based on patterned CLCs with a slanted helical axis. In this paper, the PVL is theoretically evaluated and then three reflective PVLs at red, green, and blue wavelengths (R = 605 nm, G = 532 nm, and B = 450 nm) are fabricated. Meanwhile, a simple approach is utilized to achieve 20 mm diameter and 16.5 mm focal length. The low scattering and good image quality of reflective PVL enrich these functional devices and provide promising applications to novel foldable optical systems and waveguide-based wearable near-eye displays.