Study On The Negative Reflection And Refraction Of Acoustic/optical Gradient Metasurface

Metasurfaces,ultrathin planar functional devices composed by artificially designed structures of subwavelength geometries,could tailor the shape of wavefront and control the propagation of waves within subwavelength propagating distance by utilizing single or a few layers of artificial resonant structures to locally control the scattered fields.Owing to the nature of tailoring the wavefront and controlling the propagation of waves is about the manipulation of phase,therefore,one crucial criterion to evaluate the performance of metasurfaces' building blocks is the capacity to achieve continuous control of the phase of scattered field from 0° to 360°.Gradient metasurface gains surface phase gradient by the periodic arrangement of subunits that possess continuous linear discretized phase shift ranging from 0° to 360° and could efficiently deflect the incident waves with high directivity.Different from the beam propagation control strategy governed by classical law of reflection and refraction,gradient metasurface offers a new alternative.The design principle and operating mechanism of gradient metasurfaces are simple and could efficiently reduce the volume of the devices.In this thesis,we systematically study the scattering and beam deflection properties of the acoustic and optical gradient metasurfaces by carefully investigating the interaction between incident waves and artificially designed structures,which provides the guidence for potential application in beam propagation control.Firstly,the scattering property of reflective acoustic gradient metasurface illuminated by plane acoustic waves of different incident angles is studied.Based on the gradient metasurface composed by comb-like structures whose surface phase gradient is equal to the operating wavenumber,we theoretically study its reflection behaviors under different incidences,and the all-angle negative reflection is observed.We experimentally verify the all-angle negative reflection with the gradient metasurface composed by coiling-up space units.Based on the gradient metasurface composed by “L” shape Helmholtz resonators of deep subwavelength size,we theoretically study the reflection properties of the metasurfaces of arbitrary surface phase gradient modulus,and the generalized law of reflection modified by high-order Bragg scattering and “jump-up” rule are proposed.Next,the scattering property of transmissive acoustic gradient metasurface illuminated by plane acoustic waves of different incident angles is studied.We design the transmissive acoustic gradient metasurface made of labyrinthine structures whose surface phase gradient is equal to operating wavenumber,and we further theoretically and experimentally verify the all-angle negative refraction in it.Based on the coupled mode theory,we analytically solve the all-angle negative refracted acoustic field and reveal the contribution of surface bounded wave in generating negative refraction.In addition,time-domain finite element method is further utilized to demonstrate the dynamic process of how the surface bounded waves convert to the negative refractions.The controllable asymmetric acoustic transmission realized by bilayer acoustic gradient metasurface is investigated.By selecting proper interlayer distance,the negative refraction occurring under the beyond-critical-angle illumination can be converted to negative reflection and asymmetric acoustic transmission is obtained.In order to verify that the all-angle negative reflection observed in acoustic gradient metasurface is universal,we move toward the realization of all-angle negative reflection in optical gradient metasurface.Different from the classical design strategy of metasurfaces which takes advantage of several individual subunits of discretized phase shift,here the continuous metasurface is applied to realize all-angle negative reflection and broadband negative reflection of 300 nm bandwidth in visible frequency.Continuous metasurface's building block possesses the trapezoid geometry and its constituent materials can be selected as metal or dielectrics,and perfect negative reflection can be realized by optimizing a few geometry parameters.We further utilize the ultrathin nanoantenna of metal-aluminum-indium tin oxide-metal configuration,of which the complex refractive index can be tuned by applying bias voltage,therefore,the phase and amplitude of reflected light can be actively tuned at will.Then the switchable mirror reflection-negative reflection in telecommunication wavelength is realized with 1-bit coding tunable metasurfaces.The nontrivial beam deflection obtained with optical gradient metasurface is further extended from linear optics to nonlinear optics,and we investigate the third-harmonic generation and the corresponding anomalous refractions occurred in the all-dielectric gradient metasurfaces.Firstly,we give an analytical description of the third-harmonic nonlinear polarizations excited in dielectric nanostructures and how the spin-dependent nonlinear geometric-phases are obtained under rotation operation,and we further reveal that the wave coupling among different component of total fundamental fields is essential for the appearance of different nonlinear geometric-phases.Next,we theoretically study the nonlinear geometric-phases of the thirdharmonic signals generated by silicon nanofins of different in-plane rotational symmetry.Then we theoretically and experimentally verify the anomalous refractions of third-harmonic signals generated by the all-dielectric gradient metasurfaces composed of the above silicon nanofins.