Research On Holographic Liquid Crystal Spatial Light Modulation Technology Based On Geometric Phase

Holography is the most natural three-dimensional display technology,and high-performance spatial light modulator is the key device to realize holographic display.The traditional liquid crystal spatial light modulator modulates the phase through the propagation phase.On the one hand,the thickness of the liquid crystal layer needs to be at least equivalent to a full wave plate.The thicker liquid crystal layer limits its pixel size and modulation speed,on the other hand,it is unable to carry out complex amplitude modulation.This makes it difficult to realize holographic display with large field angle and high definition based on traditional spatial light modulator.Geometric phase is the phase delay caused by the change of polarization state when light propagates in anisotropic medium.Different from the traditional propagation phase,the geometric phase modulation has nothing to do with the wavelength of incident light and the thickness of medium and amplitude modulation.Therefore,based on geometric phase modulation,it is expected to realize thinner,smaller,faster and complex amplitude modulation units,greatly improve the optical modulation ability of liquid crystal spatial light modulator,and promote the development of holographic technology towards large field angle and high definition.This thesis focuses on the mechanism and structure of geometric phase spatial light modulation:1.In view of the research status that the existing geometric phase liquid crystal spatial light modulator needs two layers of driving electrodes for driving,a reflective geometric phase liquid crystal modulation structure is proposed in this thesis,which increases the geometric phase modulation from 2 times to 4 times of the rotation angle of the liquid crystal director.On this basis,nearly all phase continuous modulation can be realized by single-layer electrode.At the same time,a multi physical field simulation analysis method combining finite element method and finite difference time domain method is established to realize the simulation analysis of geometric phase modulation and phase uniformity.2.The measurement optical path of geometric phase modulation based on laser interference is designed and built.The geometric phase modulation of reflective liquid crystal optical modulation structure is measured by comparing the gap and movement of interference fringes.The reference area is introduced into the LC cell,which eliminates the interference of air flow and platform shaking on the fringe measurement in the test process,and greatly improves the measurement accuracy.3.In view of the research status that the liquid crystal spatial light modulator cannot carry out complex amplitude modulation,a geometric phase complex amplitude modulation structure based on blue phase liquid crystal is proposed.Two groups of mutually perpendicular wall electrodes are used to form an electric field in any direction and strength in the working area.The complex amplitude modulation of light is realized by modulating the phase in the direction of the electric field and modulating the amplitude in the strength of the electric field.The electric field distribution when the specified voltage is applied to each electrode of the structure is calculated by the finite element method.Combined with the finite difference time domain method,the quantitative results of the complex amplitude modulation ability of the structure are obtained,which verifies the design idea.The results show that based on the reflective geometric phase modulation structure,the geometric phase modulation with the maximum phase range of 0~°～320~°is realized only by using a single-layer driving electrode.The numerical simulation of the complex amplitude modulation structure shows that by controlling the voltage applied on each electrode,the structure can achieve the modulation result of the beam passing through the working area with the amplitude of 0～99%and the phase of 0~°～360~°.The results show that the liquid crystal structure with geometric phase modulation has unique modulation performance compared with the traditional spatial light modulator.Compared with the traditional liquid crystal light modulation,the proposed geometric phase modulation structure has unique modulation ability,which provides a new idea for the development of high-performance holographic display spatial light modulator.