Research On Single Crystal Graphene-based Electronically Controlled Liquid-Crystal Converging Lightwave Microlens Array

The electronically controlled liquid-crystal microlens arrays(ECLCMLAs),as a micro-optical structure with electrically adjusting focus for light control application,is different from the traditional optical imaging architecture constructed by multi-lens combination to perform mechanical focusing.Based on the electro-optical characteristics of liquid-crystal materials,both the electrically adjusting and swing focus can be realized by adjusting the driving voltage signal.It has shown some remarkable characters such as miniaturization,integration,fast response and a better light control efficiency.In ECLCMLAs and liquid-crystal display devices,it is usually necessary to coat a polyimide film on the electrode material and continuously form a micro-nano-grooves on the polyimide film by mechanical friction,as an initial alignment layer of the liquid-crystal materials for presenting a functioned light control opertion.Generally,the average depth and width of the micro-nano grooves formed by rubbing polyimide film are in the tens of nanometers and sub-micron scale,respectively.The typical value of the average interval between adjacent grooves is about 0.7μm,which may cause obvious infrared diffraction crosstalk originated from the micro-nano-groove arrangement so as to affect the optical performance of liquid-crystal devices.As a new type of two-dimensional flexible nanomaterials,graphene already exhibits an extremely high transmittance in both the visible and infrared wavelength region,and also a strong anchoring effect to liquid-crystal molecules.In this paper,the graphene electrode materials and its initial orientation on liquid-crystal molecules are studied.A kind of single crystal graphene(SCG)is proposed to fabricate the electrode of the ECLCMLAs for performing initial alignment of the liquid-crystal materials,so as to eliminate two steps including traditional polyimide coating and mechanical friction orientation,and therefore realizing highly efficient converging light,at the same time avoiding the diffraction crosstalk of infrared micro-beams.The main work of this article is as follows:Firstly,based on the initial anchoring effect of graphene on liquid-crystal molecules,the surface electric field on the surface of SCG is simulated and analyzed.The property of existing three preferred alignment directions of liquid-crystal molecules along the easy axis direction over graphene is discussed.The modeling and simulation about constructing ECLCMLAs are carried out.According to the difference between the induced orientation of graphene and the friction orientation of PI,the influence of different parameters including pretilt angle and driving signal voltage under the micro-circular hole electrode corresponding to several key factors such as the director,refractive index and the phase delay distribution of liquid-crystal molecules are analyzed.Based on polarized light microscopic images,the director arrangement of liquid-crystal droplets on the surface of graphene is observed.And the distribution behaviors of liquidcrystal on the surface of SCG in six-fold radial symmetry and the difference between SCG and polycrystalline graphene for liquid-crystal molecules are analyzed.According to the standard micro-nano-process and combining the liquid-crystal orientation of graphene,SCG as an electrode materials and orientation layer,is used to fabricate a liquid-crystal microlens array based on SCG electrode,so as to simplify the process flow of liquid-crystal devices and also avoid the degradation of the device performance caused by micro-beam diffractive crosstalk,the introducing of frictional static electricity and impurities caused by traditional friction alignment operations.Finally,through the built optical test system,the SCG-ECLCMLAs is tested and evaluated in the visible(central wavelength: 531 nm and 653nm)and near-infrared spectral region(central wavelength: 980nm).The range of the focal length corresponding to abovementioned wavelength is: 1.28 mm ～ 1.49 mm,1.60 mm ～ 1.86 mm,and 1.56 mm ～ 2.48 mm,respectively.The obtained converging light pattern and then sharp point spread function indicate a good electronically controlled converging light efficiency of the SCGECLCMLAs in a relatively wide spectral region.