Generation Of Spatial Beams In Metasurfaces And Nonlinear Photonic Crystals

With the progress of science and technology,people begin to explore spatial beams with specific wavefronts.For instance,in 1992,Allen et al.found that Laguerre-Gauss(LG)beam with a spiral wavefront carries an orbital angular momentum(OAM).Compared with spin angular momentum(SAM),the OAM of light has infinite dimensions,which has broad application prospects in information encoding,nonlinear optics,particle manipulation,quantum information,and so on.Since the 1980s,scientists have also found spatial light beams with non-diffraction characteristics,i.e.,the lateral size and intensity of the beam remain unchanged during its propagation.Some non-diffraction beams feature self-reconstruction and self-acceleration.Typical non-diffracting beams include Bessel beams,Mathieu beams,and Airy beams.Bessel beams are the earliest non-diffracting beams discovered,in which the high-order ones can carry OAM.Bessel beams have been widely used in the fields of three-dimensional microscopic imaging and particle manipulation.The traditional generations of spatial light beams rely on complex optical components,which cannot meet the requirements of miniaturization and integration.In order to solve this problem,micro/nano materials such as metasurfaces and nonlinear photonic crystals are introduced.Metasurfaces have two-dimensional subwavelength structures,which is able to spatially control the phase at nanoscale precision for wavefront shaping.Nonlinear photonic crystals have spatially-variant second-order nonlinear coefficients,which can modulate the phase of the output light by changing the direction of the domain structures,thus realizing the generation of nonlinear spatial beams.In this thesis,we will focus on the generation of spatial beams in micro/nano structures,such as the generations of Bessel beams and LG beams using metasurfaces,and the generation of the non-diffraction second harmonic optical needle by using a nonlinear photonic crystal.The detailed research results are as follows:1.Generation of Bessel beams based broadband variable meta-axicons.We fabricate broadband variable meta-axicons composed of nano-aperture arrays in a metallic film.The working wavelength is from 690 nm to 1050 nm.The meta-axicons are dual-polarity.When the incident and output lights are left circularly polarized(LCP)and right circularly polarized(RCP),respectively,the metasurface is equivalent to a traditional axicon,which can re-shape the incident light to a Bessel beam.When switching the polarizations of the incident and output lights,the metasurface becomes a complementary axicon.The output light will be defocused and one can observe a virtual Bessel beam in the virtual plane.In addition,we also introduce helical phase distributions into the phase design of the metasurfaces and realize the generation of high-order Bessel beams.2.An integrated device for generation and detection of LG beams based on metasurfaces.We design and demonstrate a metasurface device which can simultaneously generate and detect LG beam.Firstly,under the LCP incident light,the phases of an LG beam and a cylindrical lens are superposed to realize the detect of the LG mode.Then,we combine the phases of the LG beam and a spherical lens.By designing the phase of the spherical lens,the metasurface can generate a focused LG beam under the RCP incident light.Finally,these two metasurface structures are staggered to form a final metasurface,which can realize a polarization-dependent integrated device for the generation and detection of LG beams.3.Generation of an OAM beam and an ultra-long second harmonic optical needle based on Li Nb O₃ nonlinear photonic crystals.The domain structures of Li Nb O₃nonlinear photonic crystals are inversed after electrical poling processes,which will introduce an additional?phase modulation into the second harmonic wave compared with the unpoled area.Firstly,based on the computer-generated holography method,we design a spirally poled Li Nb O₃crystal,which can generate a focused OAM beam at a specified focal point.This sample can also be used for the detection of fundamental waves carrying OAM.Secondly,based on the vector angular spectrum theory and the genetic algorithm,we design an annularly poled Li Nb O₃crystal.An optical needle with ultra-long depth of focus are realized under the illumination of a radially polarized fundamental wave.The length of the optical needle is about 55?_SH(?_SHis the wavelength of the second harmonic wave),and the average lateral size is about 0.45?_SH,which breaks the diffraction limit.