Three-dimensional Structured Liquid Crystal Geometric Phase Optical Elements And Their Applications

Compared to refractive optical elements and dynamical phase based conventional diffractive optical elements,planar optical elements based on geometric phase can increase the degree of the freedom of the system,and also have many advantages such as small volume,light weight,high precision and high efficiency.Therefore,geometric phase based diffractive optical elements and their applications have received extensive attention in recent years.Liquid crystal,as a unique self-assembled soft-matter functional material,thermodynamically situates between normal isotropic liquid and three-dimensionally ordered crystal.It plays an irreplaceable role in displays and photonics.Geometric phase optical elements based on liquid crystals can modulate the polarization states,phases,and wave vectors of light and therefore can be widely used in optical micro-manipulation,communication,information processing,virtual reality,augment reality,and other places.With the development of alignment technology of liquid crystals and the innovation of fabrication processes of liquid crystal devices,the spatial arrangement of liquid crystal orientation has the greater freedom.Furtherly,complex 3D nanoscale-ordered liquid crystals can not only be used to form some geometric phase elements with specific functions,but also offer more freedom for elements to manage the spectrum,view-angle,and even other parameters of light.In this thesis,based on the transmissive geometric phase from nematic liquid crystal and the reflective geometric phase from cholesteric liquid crystal,with a combination of photoalignment technology and liquid crystal device fabrication,the complex 3D structured liquid crystal optical elements with specific functions are designed and fabricated to solve some issues in light shaping.In addition,a near-eye display system with an expanded eyebox based on broadband liquid crystal polarization gratings is also studied.The main results are listed as follows:(1)In terms of designing the micro-structures of liquid crystals,the photoalignment technology is utilized to realize the arbitrary distribution of liquid crystal azimuthal angle on the surface of the device through using the polarization holographic method such as laser direct writing,polarization interference,digital micro-mirror device.In addition,liquid crystal directors can be controlled from bottom to top based on the self-assembled character of liquid crystal and the anchoring force on the surface of liquid crystal polymer film.Such method enables liquid crystal directors to be arranged in 3D directions,so as to achieve more complex micro/nanoscaled structures of liquid crystals.(2)The geometric phases can be obtained from nematic liquid crystal and cholesteric liquid crystal and their spin to orbital angular momentum conversion processes can be conducted.However,vortex beams with different topological charges from nematic liquid crystal q-plate cannot be spatially separated and the transmissive circular polarized light from patterned cholesteric liquid crystals cannot conduct the spin to orbital angular momentum conversion.In view of the above issues,a novel liquid crystal vortex generator with high efficiency is designed.Such a vortex generator combines liquid crystal polymer film and chiral nematic liquid crystal,which makes both transmitted light and reflected light generate vortex beams with same topological charges of orbital angular momentum.(3)Because cholesteric liquid crystal can selectively reflect circular polarized light and the reflected light can obtain geometric phase,two opposite geometric phases are obtained and even the polarization states of added reflected light can be converted through combining two opposite-handed cholesteric liquid crystals.In this way,an arbitrary Bragg reflective linear polarization distribution pattern can be obtained by controlling the azimuthal angle of liquid crystal directors on the device surface.The device can generate vector beams in Bragg reflection bandgap or even a broader band range.It can be utilized in high-resolution imaging,focusing,and information processing.Our proposal also provides a basis for generating arbitrary vector vortex beams on the high order and hybrid order poincare sphere.(4)Based on the optical principles of liquid crystal polarization lens and polarization grating,broadband polarization grating polymer films with two-layer liquid crystal twisted structures are designed.When the linear polarized light is used to illuminate the device,the diffraction efficiency in zero order,positive first order and negative first order is 33.3% in the whole visible range.In addition,a 2D diffraction grating is designed based on two broadband polarization gratings and one quarter wave plate.The 2D grating is inserted to enlarge the eyebox of Maxwellian-type full-color near-eye displays.The eyebox is expanded from 3 mm × 3mm to 9 mm × 9 mm,which meets the requirement of the current near-eye displays.Such proposal is simple,robust,low-cost,and high-performance.Its potential applications for near-eye displays is foreseeable.