| The artificial manipulation of the amplitude,phase,polarization,wavelength and other parameters of electromagnetic waves in various frequency bands is the physical basis of modern optoelectronics and photonics.Optical vortex(OV),featured by a helical phase front,carries unique characteristics of propagation dynamics and the orbital angular momentum(OAM)of light.It has attracted ever growing attention over the past few years,especially in enhancing the light-matter interaction,breaking the diffraction limit of traditional imaging,increasing the capacity of optical communications systems and the dimension of quantum infonnatics.Along with many specific problems raised in practical applications and the rapid development of micro/nano-photonics,the control technology of OVs is constantly developing under the joint promotion of cutting-edge applications and basic researches.Liquid crystals(LCs)have both the optical/dielectric anisotropy of a crystal and the fluidity of a liquid.As an excellent electro-optic material,its reliability has been well proved in the field of information displays.In this thesis,focusing on the efficient generation,active control,array modulation and broadband detection of OVs,we systematically and deeply research on the design and fabrication of various LC domain structures and their interaction mechanisms with OVs.By fully exploring the rich stimuli-responsive features of this soft matter,we realize flexible light controlling to address some key issues in the generation,manipulation and detection of OVs.The main research results are as follows:(1)About the fabrication technique of LC domain structures:Through optimizing the previously developed digital micro-mirror device(DMD)based micro-lithography setup and employing a polarization-sensitive alignment agent SD1,we propose a dynamic photo-patterning technology.Based on the combination of a step-by-step partly overlapping exposure and the synchronous polarization rotation,accurate,arbitrary and reconfigurable LC azimuthal orientations can be obtained.The collaboration of the "top-down" photo-patterning process and"bottom-up" LC molecules self-assembly pushes the tailoring of nematic and cholesteric LC domain structures into an unprecedented level.(2)About the efficient generation of OVs:Firstly,we introduce the concept of geometric phase to endow the traditional fork grating with a periodically space-variant LC domains.Accordingly,an LC forked polarization grating is proposed,capable of high-efficiency generation of arbitrary OVs.Corresponding modes carrying integral or fractional topological charges,and zero or non-zero radial indices are demonstrated with a conversion efficiency up to 98.5%,exhibiting unique features of polarization control,electrical switching,and reconfigurability.Then,on the basis of the polarization-guiding effect of the twisted nematic(TN)LCs,we propose an LC polarization converter for arbitrary vector beam generation by imprinting different domain patterns into respective substrates.In particular,high-order and multi-ringed vector beams are presented.The obtained LC converters are further utilized as polarization masks to implement vector-photoaligning,facilitating the preparation of LC q-plates.(3)About the array modulation of OVs:Firstly,we propose a design of LC Dammann vortex grating composed of alternative orthogonally planar-aligned LC domains by referring its special phase structure into the LC photo-patterning.Various one/two-dimensional OV arrays,including a variety of high-order OAM modes,are demonstrated in high quality and good energy uniformity.Other merits of excellent polarization independency,electrical tunability,and wavelength tolerance are exhibited as well.Next,we propose a concept of digitalized geometric phase by encoding the conventional spiral geometric phase with the specially designed binary phase of(circular)Dammann grating.Higher-order Poincare sphere beam lattices with square and hexagonal symmetry are experimentally created using space-variant LC domains,enabling parallel optical spin angular momentum and OAM encoding.Besides,both single-and multi-ringed perfect higher-order Poincare sphere beams are achievable.Via selecting the incident spin,any point on the(perfect)higher-order Poincare sphere can be obtained,including specific cases of(perfect)vector beam and(perfect)OV.These compact LC devices show advantages of simple configuration,high efficiency and uniform energy distribution.(4)About the broadband generation and detection of OVs:Firstly,we propose a digitalized self-organized LC chiral superstructure to realize a multifunctional OV processor.Through further exploring the polychromatic and spin-determined reflective phase modulation from cholesteric liquid crystals,up to 25 different OVs are directly extracted with equal efficiency over a wavelength range of 116 nm in visible band.The multiplexed OVs can be detected simultaneously without mode crosstalk and the transmitted mode remains unchanged,permitting a broadband,large-capacity,and in situ method for parallel OAM demultiplexing.Moreover,we introduce a photo-responsive chiral molecular switch and a static dopant with opposite handedness to LCs to form a light-controlled chirality invertible superstructure.With different photo-patterning LC domains,several active planar optical elements are produced and have continuously tunable working spectra and light-triggered function transformations.Particularly,polychromatic OVs are reversibly tuned in an ultra-broad band over 1000 nm covering green to telecomm region.Corresponding spin-to-OAM conversion can be driven by light,verifying photo-invertible OAM. |
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