Study On The Generation Method Of Near-Infrared Vortex Beams Based On Geometric Phase Metasurfaces

Vortex beam known as orbit angular momentum(OAM)beam,is a beam propagating singularly around its optical axis expressed as exp(il?),where?is the azimuthal angle and l is the topological charge of the vortex beam,which means that each photon carries the certain orbit angular momentum of l.The OAM beam has characteristics of spiral wavefront,doughnut-like intensity distribution and propagating independently which has attracted a great deal of interests from many applications,such as optical manipulation,microsensors,optical metrology,quantum information processing and optical communication.The traditional optical devices that produce OAM beams are spiral phase plate,spatial light modulator and so on.With the development of integrated optical system,the requirements for devices are becoming smaller,lighter and more flat.The traditional converters suffer from the drawbacks,such as bulk size,non-flat design which make them difficult for optical integration in many applications.In recent years,the theoretical design,manufacture and application of metamaterials with various optical functions have attracted the attention of scientists in many fields of physics.Metasurfaces are 2D version of metamaterials composed of periodic subwavelength units,which can cause phase mutation of the light wave in a thickness smaller than the input wavelength.Therefore,it can modulate the parameters of light wave in subwavelength resolution,such as intensity,phase,polarization,etc.which is very suitable for building the new type of photon device and solving many problems induced by the traditional converters.Based on the geometric phase principle,a series of researches have been carried out on the design of metasurfaces and the possibility of using metasurfaces as half-wave plate,vortex beam converter,and multi-function device in this dissertation.The main contents are listed as follows:1.Based on the geometric phase principle,a new type of plasmonic metasurface is designed,which is composed of rectangular holes etched on an ultra-thin gold film.Theoretical research and simulation results show that both the scalar vortex beam with the topological charge of±2 and the vectorial vortex beam with the polarization order of±1 are generated when the circularly polarized light is incident on the metasurface vertically in the near-infrared band.The analysis predicts that the device has a good tolerance and the sample is successfully developed by template transfer method.The sample is tested on the self-built optical platform,and the results are in good agreement with the theoretical and simulation results.At 1550 nm,the transmittance of the metasurface is nearly 40%.As a scalar vortex beam converter,the intensity ratio of the vortex beam is 0.5087.As a vectorial vortex beam converter,the intensity ratio of the vortex beam is 0.9935.Although the transmittance and chromatic aberration of the metasurface need to be further improved,this work provides a new idea for the development of new high-efficiency vortex beam converters.2.Based on the geometric phase principle,the all-dielectric metasurface is designed,which is composed of nano Si pillars as emission units distributed in 21×21 array,and Si O₂ as the substrate.The working wavelength of the device is in the near-infrared range of 1.5?1.6?m.When the circularly polarized light is incident on the metasurface vertically at 1550 nm,the vortex beam with the topological charge of±1 is generated and the mode purity of the vortex beam is 90.66%.When the linearly polarized light is incident on it vertically at 1550 nm,the radially and azimuthally polarized beam are generated respectively,the purity of the azimuthally polarized beam is 92.52%,and that of the radially polarized beam is 91.02%.The metasurface also can act as dual-channel data encoding and decoding devices by introducing the phase gradient into the design.The above results are verified by numerical simulation.The designed metasurface has potential in application,such as optical communication,optical manipulation.3.The Gap-Plasmon metasurface arranged in 11×11 array of units is designed.Each unit is composed of three layers.The top layer consists of two elliptical silver nanopillars to form an L-shaped antenna.The intermediate layer is Si O₂ and the bottom layer is silver film.The device works in the near-and mid-infrared range of2.7?4.2?m.The designed metasurface can act as a half-wave plate with chromatic aberration-free under the the linearly and circularly polarized incident light,and the polarization conversion rate is about 85%.Based on the geometric phase principle,the units rotated by certain angles,the metasurface can act as a vortex beam converter without chromatic aberration and produce the vortex beam with the topological charge of±1 under the circularly polarized incident light,and the mode purity of the vortex beam is more than 80%.The vortex beam can be separated from the incident beam,which makes the metasurface more practical by introducing the phase gradient into the design.The above numerical simulation results are in good agreement with the theoretical results.This research has theoretical significance for the development of photonic devices without chromatic aberration in the near-infrared band.