Study On Electro-optical Modulation Properties Of Holographic Liquid Crystal Gratings

Holographic Polymer Dispersed Liquid Crystal(H-PDLC)gratings benefit from the electronic control properties of liquid crystals and can realize the conversion from incident light diffraction to transmission under an external driving electric field.It has the advantages of simple process and so on,therefore it has certain application value in holographic storage,holographic display,optical communication and so on.The H-PDLC grating mainly uses the orientation rotation of the liquid crystal molecules in the grating under an external driving voltage,which changes the refractive index modulation of the grating,thereby reducing the diffraction efficiency of the grating to realize the switching of the working mode.However,the liquid crystal molecules are distributed in the droplets wrapped by the polymer matrix,and due to the anchoring energy between the substrate and the polymer,it is more difficult to change the orientation than the general liquid crystal photonic devices,so the driving voltage is higher,which to a certain extent restricts the extensive development of its application.Therefore,in this paper,the two key factors affecting the H-PDLC grating,diffraction efficiency and driving voltage of the grating are studied by simulation and experiment.On the basis of preparing a high diffraction efficiency grating,to fabricate low driving voltage gratings the effect of different additives on the driving voltage of the grating is also studied.In this paper,the simulation of the grating with high diffraction efficiency is carried out at first.On the basis of introducing the influencing factors of the diffraction efficiency of the H-PDLC grating,the influence of the refractive index modulation on the diffraction efficiency of the grating is mainly studied.Secondly,the effect of liquid crystal orientation on the diffraction efficiency of the grating is studied.If the orientations of the liquid crystal of the H-PDLC grating before and after the application of the driving electric field are parallel to the grating vector and perpendicular to the substrate,the maximum diffraction efficiency change will be obtained under the p-polarized light test conditions.The paper also studies the preparation process of H-PDLC grating with high diffraction efficiency.The diffraction efficiency is prepared under the parameters of exposure intensity of10.70m W/cm~2,exposure time of 100s,liquid crystal cell thickness of 20μm,and beam angle of about 30°.The grating diffraction efficiency is up to 96.3%;the angle dependence and polarization dependence of the diffraction efficiency of the grating are studied.On this basis,the influencing factors and fluctuation curve of the grating driving voltage are analyzed and studied in this paper.By studying the mechanism of surfactant addition,two kinds of ionic and non-ionic surfactants are selected to be added.Compared with the electronic control effect of nano-silver particle grating,the grating doped with 6%nonionic surfactant SPAN 80 achieved better results,and the threshold driving electric field of the grating was reduced from 15V/μm to 1.37V/μm.The results also demonstrate that exposure times that are too long in the fabrication will lead to overmodulation of the grating,causing fluctuations in the electrical modulation.