The Light-reconfigurable Multifunctional Diffractive Optical Element Based On The Self-assembled Microstructures Of Photosensitive Cholesteric Liquid Crystals

Liquid crystal is an intermediate state between liquid and crystal.It has both the anisotropy of crystal and the fluid characteristics of liquid.As a special optical material,the soft material properties of liquid crystal give it responsive characteristics to external stimulus,such as light,electricity,magnism,temperature,mechanical force,resulting in the change of liquid crystal molecule vector,which brings liquid crystal optical and electrical properties to obtain a variety of applications.The research in this thesis is based on the properties of photosensitive chiral nematic liquid(cholesteric liquid crystal)with one-dimensional self-organized helical structures.Based on the photoalignment surface anchoring technology and the photosensitive characteristics of azobenzene materials,the amplitude-phase compositive diffractive element is realized with the advantages of high diffractive efficiency of the phase diffractive element and multiple modulations of reflected light.The type of diffractive element can be easily implemented through light stimulus.Based on the photoalignment anchoring technology,the photosensitive chiral nematic liquid crystals can be uniformly anchored on the surface.Through two-step ultraviolet exposure with a patterned photomask,the liquid crystal molecules with the periodically graphical orientation can be obtained,and then the wavefront geometric phase of the reflected light can be modulated.Based on this,phase diffractive elements were fabricated.Due to the doped photosensitive azobenzene chiral material,the molecular configuration changes from the trans-isomer with higher helical twisting power(HTP)to the cis-isomer with lower HTP under UV irradiation,and the reverse process will occur under the irradiation of visible light(green light).The light-modulated HTP implements the helical pitch modulation of chiral nematic liquid crystal,resulting in the adjustment of Bragg reflection waveband.Based on this principle,the phase diffractive element with a selective reflection band is prepared whose working waveband can be flexibly modulated through UV/VIS light.According to the photo-isomerization of photosensitive azobenzene material,the bottom-up self-organized helical structures of CLC with light-tunable property lead to the modulation of reflected light intensity(amplitude).Combining the top-down substrate anchoring based on photoalignment,the geometric phase of reflected light is also modulated and the amplitude-phase compositive diffractive optical element is fabricated.This element has the unique characteristics of optical-switching,easy-operated and repeatedly erasable.In this experiment,the unique effects of modulations and diffractions of the reflected light are systematically analyzed based on diffractive theory.In conclusion,the light reconfigurable multifunctional diffractive optical element based on photosensitive multistable chiral nematic liquid crystal provides a new idea for ’smart’ optics,which can be applied in the research fields of photophysics,photochemistry,all-optical signal processing and optical manipulation.