Study Of Reconfigurable Directional Beaming And Ultrabroadband Airy Optical Beams Generator Based On Metasurfaces

Metasurfaces are a new type of artificial electromagnetic materials that the arbitrary effective permittivity or permeability can be realized by choreographed subwavelength structures,composed of the periodic or quasi-periodic arrangement of two dimensional subwavelength structures,which break the limitation of the natural materials and introduce a new design freedom for controlling electromagnetic waves.Due to its compact structures and powerful controls of electromagnetic waves,metasurfaces show the tremendous potential of application in many areas.However,there are still some difficulties in the research field of metasurfaces that restrict the development and practical use of metadevices,including lack of adjustability and low efficiencies.In this thesis,we have studied the reconfigurable multi-frequency and wide-angle directional beaming of light from a subwavelength metal slit with graphene metasurfaces,as well as ultra-broadband highefficiency airy optical beams generated with all-silicon metasurfaces,providing an alternative scheme for improving the adjustability and efficiency of metasurfaces.The main contents of research are as follows:(1)Thanks to the adjustability of the graphene's chemical potential,graphene is introduced in the design of metasurface to overcome the limitation of narrow working bandwidth and lack of adjustability in the reported directional beaming.The graphene can be regarded as the single layer metal in the spectral range of interest,threrfore,the graphenedielectric-metal unit with adjustable magnetic resonance is proposed in this thesis.After the incident lights pass through the slit,a portion of energy is coupled to surface plasmon polariton(SPP)propagating on the surface of gold and dielectric,while the remaining energy is directly diffracted into air uniformly.The proper phase gradient can be freely designed at the both sides of metal subwavelength silt to compensate wave vector mismatch between SPP and propagating wave(PW).Thereby,when the SPP at both sides of silt are converted into the propagating waves in same direction,they interfere constructively with the light emitted directly from the subwavelength metal slit,forming the directional collimated beam.Thanks to the negligible interaction between the vertically stacked graphene ribbons,the working bandwidth can be broadened by use of double layers of the graphene ribbon array.The reconfigurable multi-frequency and wide-angle directional beaming is verified by the numerical simulations conducted by the 2D finite-difference time-domain(FDTD)method within the working bandwidth of 10.6?29.1 THz.(2)In the study of ultra-broadband high-efficiency airy optical beams generated with all-silicon metasurfaces,a design strategy of constructing broadband and high-efficiency Airy optical beam generator by taking advantages of the high birefringence and transmission efficiency of the silicon nano-pillar is proposed,in order to overcome the limitation of narrow bandwidth and/or low efficiency of reported Airy beam generators based on metamaterials/metasurfaces.The relationship between the transmission amplitude/phase of electric field,whose polarization is orthogonal to that of incidence,and the orientation angle of the nano-pillar is deduced,when the linearly polarized incident light illuminates the birefringent nano-pillar.It indicates that the normalized amplitude can be tuned from 0 to 1by the orientation angle of the nano-pillar and the transmission phase experiences a ? phase shift when the sign of orientation angle changes,regardless of light wavelength.The ultrabroadband high-efficiency airy optical beams generator is constructed by painstakingly configuring the orientation angles of silicon nano-pillars to satisfy the local amplitude and phase of the Airy function,simultaneously.The resultant Airy beam generator is numerically verified by FDTD method within the spectral range from 0.7 to 2.3 ?m and the maximal transmission efficiency reaches 76.8% at the central wavelength.The performances of the designed generator are measured within the operation bandwidth from 1.1 to 1.7 ?m,which are highly consistent with the theoretical predictions and simulation results.The results may power up viable opportunities for practical devices and applications of Airy beams based on meta-devices.