Research On The Generation And Regulation Of The Metasurface Optical Orbital Angular Momentum

The orbital angular momentum(OAM)beam is a special vortex beam with helical phase structure,and its phase factor can be expressed as exp(i??),where ? is the azimuth,? is the topological charge value,and each photon in the OAM beam carries a specific orbital angular momentum value ??.Because of its special phase structure and orthogonal mode,it has great potential application value in improving the communication capacity,communication rate and communication security of quantum communication and optical communication.Moreover,more and more attention has been paid to the use of a two-dimensional planar periodic artificial material — — metasurface to regulate the electromagnetic wave within the subwavelength,so as to generate OAM beam to meet the needs of future communication development.This paper focuses on the generation and regulation of the superposition state of OAM and flat power spectrum of OAM superposition state based on metasurface.Compared with other metasurface OAM generators,OAM generators with more superimposed OAM modes and more stable power fluctuations are designed.The main research and results in this dissertation are listed as follows:1.A kind of metasurface device which can generate OAM superposition beam is designed.Because the OAM beam has spiral wave front phase,in the use of metasurface geometric phase control method,and through the reasonable setting the function relation between azimuth and dipole cell rotation angle of the metasurface,to form a spiral phase gradient.The phase field with the quadratic power exponent phase can be obtained by the incident polarized beam through the metasurface composed of dipole cell array with spatial variation.The phase factor can be expanded by Fourier series to obtain the OAM superposition states possessing tens of OAM modes being evenly spaced by topological charge number 1,the relative power variation between modes within the flat range of superposition power spectrum is less than 3d B and can work in a wide band.2.A metasurface generator with OAM superposition state and power fluctuation less than 1d B is designed.Based on the idea of multi-function control of metasurface structure partition,the structures of two sets of metasurface dipole units with different rotation directions are uniformly crossed to achieve multi-function combination,the linear superposition of the quadratic power exponent phase is realized to realize phase increment modulation of the incident light.By Fourier series expansion of the phase factor obtained,the power spectrum with relative power fluctuation of each mode of OAM superposition state less than 1d B can be obtained.Moreover,under the incident light of different circular polarization states,the power spectrum of the superposition state OAM can be separated from each other,so as to realize dynamic polarization control and work effectively in the wavelength range of 635nm-730 nm.