Broadband Optical Devices Via Patterned Double-Layer Reverse-Twisted Liquid Crystal

In modern optics,planar optical elements based on geometric phase have the characteristics of high integration and compact volume.At the same time,multidimensional optical parameters can be manipulated via a single element.Therefore,they are widely concerned by researchers and can be used in many fields such as optical field manipulation,holographic imaging,beam deflection and optical array generation,etc.Geometric phase is caused by the change of polarization state and depends on the evolution path,so it is usually generated by inhomogeneous anisotropic materials.Liquid crystal geometric phase elements have kept being a focus of recent research.Due to their photoelectrically adjustable characteristics,they are widely used in imaging,optical communication,special beams generation and other fields.However,the geometrical phase elements of nematic liquid crystal suffer from serious wavelength dependency,that is,the maximum diffraction efficiency can be achieved only when the half-wave condition satisfied for incident light.In order to overcome this shortcoming,this paper introduces a double-layer reverse-twist liquid crystal structure to realize broadband liquid crystal polymer elements.Then,combining with a pattern of geometric phase,various broadband high-efficient liquid crystal elements are fabricated,which are easy to manufacture with simple structures.It provides a very promising strategy for the broadband and efficient polarization manipulation.The main research results are as follows:1.With a DMD system,the pattern structure of polarization grating is accomplished by photo-aligning technique.Under the half-wave condition,the polarization grating diffraction efficiency of the zero order is 0,thus the maximum bandwidth can be optimized,which is determined by thicknesses of both liquid crystal layers and respective concentrations of chiral agents.On this basis,a broadband liquid crystal forked polarization grating is prepared too.Meanwhile,traditional samples of the two kinds of grating are prepared according to the half-wave condition of 633 nm for comparations.It was measured that the diffraction efficiency of the broadband polarization grating and the forked polarization grating could reach more than 95% in the band of 530 ～ 930 nm,and the diffraction beams of the forked grating carry corresponding topological charges.However,the diffraction efficiency of ordinary samples reach maxima when the incident wavelength is 633 nm,and the diffraction efficiency decreases rapidly when the wavelength deviates from 633 nm.2.On the basis of the previous work,we further design and introduce the geometric phase of the 2 × 2 Dammann vortex grating and the hexagonal Dammann q-plate.Two broadband vortex array generators are fabricated with the double-layer reverse-twist liquid crystal structure,which can generate 2 × 2 rectangular array and hexagonal lattice array of optical vortices,respectively.Within the bandwidth of 530 ～ 930 nm,the sum efficiencies of all first orders exceed 40.1% and 51.0%,respectively,and remain stable.Compared with traditional liquid crystal Daman vortex grating/Daman q-plate,whose diffraction efficiencies reach maxima only at half-wave conditions,and then decrease rapidly after the wavelength mismatching the half-wave condition.This work paves the way for broadband and efficient beam shaping,wavelength division multiplexing compatible mode coding,and high-density information storage and encryption.