Optically imprinted polarization gratings with a twisted nematic liquid crystal as a photomask

Liquid crystal Pancharatnam–Berry phase optical elements (LCPBOEs) are becoming increasingly attractive elements to be used to enhance near-eye displays or manipulate light beams, owing to their flatness and circular polarization-dependent behavior. The optical imprinting technique that is one of the commonly used methods for fabricating LCPBOEs is considered to be the most promising technique for mass production at a low cost, but the design of the photomask must be improved. In this work, a photomask that is based on a twisted nematic liquid crystal polarization grating (TNLCPG) cell, acting as a broadband linear polarization rotator, is designed. This TNLCPG cell, with a predefined alignment profile of a polarization grating, can convert linearly polarized light into a light field with spatially varying linear polarization and exhibits broadband operation over the entire visible spectrum. The designed TNLCPG photomask is used to demonstrate a liquid crystal polarization grating (LCPG) film using a white flashlight as a writing source. A high diffraction efficiency of greater than 95% is obtained. With its simple design and fabrication, the proposed TNLC-based photomask can potentially be used in the fabrication of LCPBOEs.