Generating Optical Vortex And Terahertz Vortex Via Liquid Crystal Microstructures

In recent years,optical field modulation plays a more and more important role in the development of optics,through which the amplitude,polarization,phase and wave front of Gaussian beams can be modulated and special optical beams can be achieved.Among them,optical vortex has wide application prospect in optical communication,quantum informatics,optical manipulation and ultra-resolution microscopic imaging due to its unique helical wave front and orbital angular momentum.With the rapid development of elements for terahertz modulation,modulating it to generate special wave fields has gained more and more attention.And terahertz vortex which combines the merits of terahertz and orbital angular momentum holds great potentials in communication,sensing and imaging.Liquid crystal possesses optical anisotropy,dielectric anisotropy and excellent electro-optical characteristics in visible,infrared,even terahertz and microwave band.It has been widely used in flat panel display.And by special techniques,special liquid crystal microstructures can be fabricated for modulating different parameters of optical fields.In this thesis,we employ two kinds of liquid crystals and fabricate different liquid crystal microstructures with different methods to achieve optical vortex(array)generator with fast response and terahertz vortex generator.The major results are summarized as follows:We fabricate patterned electrodes using computer generated fork grating hologram as lithography masks.Polymer stabilized blue phase liquid crystal is introduced and driven by patterned electrodes in vertical field switch mode.Fork grating phase profiles are formed to generate optical vortices.For the incident wavelength of 632.8 nm,a maximum conversion efficiency of 33%for optical vortex in ±1st order is obtained when the applied voltage is 95 Vrms.Switch between voltage on and off states results in switch between vortex on and off states,and a fast response on/off time of 364 ?s/571?s is measured respectively for 50 Vrms.The voltage-dependent efficiency curves of 1st order at different incident polarizations verify the polarization independence of the blue phase liquid crystal fork grating.It is the first time in the world to realize fast response fork grating based on blue phase liquid crystal,supplying a new way to generate and modulate optical vortex.By introducing the concept of Dammann grating,we design Dammann vortex grating and fabricate the corresponding patterned electrodes.Blue phase liquid crystal is driven by the patterned electrode in vertical field switch mode and a 1 × 7 vortex array is generated.Different diffraction orders have different topological charges which correspond to different orbital angular momentums but the energy distribution of them approximates with each other.The diffraction efficiencies for high orders are obviously higher than those of a conventional fork grating.Besides,the blue phase Dammann vortex grating also keep the features of fast response and polarization independence.As for the limitation of material and fabrication,we propose the possible optimizing and resolving strategy.The fast switchable Dammann vortex grating based on blue phase liquid crystal may boost the applications of optical vortex in informatics,beam shaping and optical communication.We introduce a large birefringence liquid crystal and fabricate liquid crystal terahertz q-plate via multi-domain photoalignment technique based on azo-dye SD1 and dynamic micro-lithography setup.Terahertz vortices with different topological charges are generated.We also demonstrate the feasibility of electrically tunable terahertz q-plate.The current limitations are pointed out and practical optimization methods are given.It is the first time to realize terahertz vortex generation utilizing liquid crystal,which may prompt applications of terahertz vortex in THz communication,sensing,imaging,etc.Based on liquid crystal terahertz q-plates,we introduce the concept of polarization grating and fabricate liquid crystal terahertz forked polarization grating which integrate the polarization selective diffraction of polarization gratings and the vortex generation of fork gratings.Terahertz vortices are generated and separated from undesirable non-vortex components,eliminating the extraction process.Resultant terahertz vortex beams with opposite circular polarization and topological charges only exist in-1st and+1st orders,which can be controlled by incident circular polarization.The liquid crystal terahertz forked polarization grating provides an easy and efficient strategy for generating,polarizing and separating terahertz vortex beams and has referential significance for designing other terahertz beam shaping devices based on liquid crystal.