Design And Realization Of High-efficiency,Wide-spectrum Liquid Crystal Polarization Grating

The liquid crystal polarization grating(LCPG)is a new type of geometric phase element.It can modulate the phase and polarization state of incident light by adjusting the spatial distribution of the liquid crystal directors.In theory,it can achieve 100%diffraction efficiency for incident circularly polarized light.It has broad application prospects in laser scanning,polarization imaging,optical fiber communication,liquid crystal display and other fields.However,the overall diffraction efficiency of current LCPG is not high enough,and the spectral range is relatively narrow,which limits the application of LCPG in various optical systems.Therefore,based on these problems,this paper studies the structural design of the LCPG with high diffraction efficiency and a wide band,and fabricates the LCPG with high diffraction efficiency and a wide band.First of all,this thesis analyzes the main factors that cause the energy loss of the LCPG,including the intrinsic energy loss caused by higher order diffraction,the scattering loss caused by the grating topography defects and the Fresnel reflection caused by the mismatch of the refractive index of multiple interfaces.The mechanism of the above-mentioned energy loss was studied,and the preparation process was optimized based on this.First,the high-order diffraction was eliminated by optimizing the exposure light path,and then the probability of defects was reduced by the control of the critical thickness of the liquid crystal layer.Finally,the Fresnel reflection is reduced by the method of coating a multilayer antireflection film,so as to realize the preparation of the LCPG with high diffraction efficiency at a single wavelength.Then,this thesis studies the twisted structure,and designs a new twist angle and thickness measurement method for twisted LCPG based on Jones matrix.This method starts from the calculation method of the transmittance of the TN box,and simplifies the complexity of the transmittance function by making the polarizer and the analyzer parallel to each other.The similarities and differences between the twisted liquid crystal film and the twisted LCPG are analyzed,and the measurement results based on the twisted liquid crystal film can be used to obtain the twist angle and thickness of the twisted LCPG prepared under the same conditions.Moreover,the method of rotating the liquid crystal cell improves the repeatability and accuracy of the twist angle and thickness measurement process.Next,this thesis analyzes the multi-layer twisted structure of the wide-spectrum LCPG based on the PB phase theory.Starting from the Muller matrix of the twisted liquid crystal film,the polarization conversion process of the incident light in the twisted liquid crystal film is obtained.At the same time,the Stokes parameter of the polarization state is converted into the coordinates on the Poincaré sphere,so that the process can be described intuitively and clearly on the Poincaré sphere.It can also be found that the twisted LCPG can be equivalent to a series of twisted liquid crystal films with the initial orientation angle rotated during the conversion process between circularly polarized light at this time.Therefore,based on the Muller matrix of the twisted liquid crystal film,the final structure of the twisted LCPG that can realize a wide band can be obtained through optimization.Finally,this thesis is based on the linear relationship between the concentration of the added chiral agent and the twist angle under the same thickness.The linear function can be obtained by measuring the twist angles at several chiral agent concentrations,so that the required twist angle and thickness of each layer of the twisted LCPG can be obtained to achieve the required concentration of the added chiral agent.Thus,the preparation of a wide-band LCPG is realized based on the obtained multi-layer twisted structure.Through the research of this paper,this kind of wide-spectrum,high-diffraction efficiency element is expected to be truly practical in the field of display and optical communication.