Research On Mechanism And Performance Of Wavefront,Polarization And Surface Wave Coupling Devices Based On Metasurfaces Structures

As a kind of two-dimensional metamaterials composed of subwavelength-thick artificial units,metasurfaces have attracted wide attention due to their extreme abilities to control the amplitude,phase and polarization state of electromagnetic waves(EWs)at the subwavelength scale.By using metasurfaces to locally control the spatial distributions of the amplitude,phase,and polarization of EWs,such functions as abnormal refraction/reflection,focusing,holographic imaging,and special beams generating have been achieved.However,there are still some difficulties in the metasurfaces research field that restrict the practical use of devices,including low diffraction efficiencies,dependence on high refractive index materials,and lack of adjustability.In response to these problems,this thesis proposes solutions based on hyperbolic metamaterial(HMM)and graphene.In addition,using the ability of the metasurfaces to locally control the phase of the radiated EWs,high efficiency directional beaming of EWs emitted from the subwavelength aperture has been demonstrated in this thesis.The main research contents and innovations of this thesis are listed as follow:(1)In the study of HMM-based wavefront and polarization manipulation devices,considering the problem that metasurface areas suffer from low transmission efficiency and relying on the transparent materials with high refractive index,this thesis proposes that high-efficiency wavefront and polarization manipulation can be realized by using the novel HMM waveguide array based metasurfaces.First,we calculated the dispersion relations of the HMM waveguide modes and proved that,compared with dielectric metasurfaces,HMM waveguide can offer more tunable dispersion and a higher equivalent refractive index without using high refractive index dielectric materials.In addition,thanks to the coupling between the high-order diffractions and the fundamental mode of the HMM waveguide array,the high transmittance and high effective refractive index can be obtained simultaneously by the HMM waveguide array.Therefore,the HMM waveguide array can efficiently control the phase and wavefront of the transmitted EWs by changing structural parameters of the waveguide.We designed,fabricated and measured beam deflectors and focusing lenses at microwave frequencies,where both the simulation and measured results validated the devices' ability to deflect or focus incident EWs.To further demonstrate that HMM waveguide based metasurface is suitable for any frequency,we designed and simulated the polarization-independent beam deflectors and focusing lenses at the wavelength of 1550 nm.The conversion efficiency of the polarization-independent beam deflector can be up to 71%,far beyond those of single layer metal metasurfaces and comparable to dielectric metasurfaces.We have also demonstrated that the HMM waveguide with a rectangular cross section presents a huge mode birefringence effect.Based on this mode birefringence effect,a subwavelength-thick terahertz polarization manipulation device with the functions of quarter-and half-wave plates has been designed.This polarization manipulation device has performance merits,including high transmission efficiency,ultra-compactness,and tunable birefringence,offering a promising approach to manipulating the polarization states of EWs.(2)In the study of using gradient metasurfaces to realize the directional beaming of EWs from a subwavelength metal aperture,this thesis proposes a new type of high efficiency directional beaming scheme based on gradient metasurfaces,to overcome the problem of low diffraction efficiencies of directional beaming devices based on planar gratings.The low diffraction efficiencies of the directional beaming devices based on planar gratings are usually caused by the high-order diffractions of the gratings.Unlike planar gratings,gradient metasurfaces can freely select the needed diffraction order and suppress other orders by their phase gradients.Therefore,the low diffraction efficiency issue of the planar grating-based directional beaming devices can be overcome by replacing the grating structures on both sides of the aperture with precisely designed gradient metasurfaces.To validate this design concept,we designed,fabricated and tested three samples with different beaming angles in the microwave frequencies.Both the experimental results and the simulation results confirm that the diffraction efficiencies of the metasurface-based directional beaming devices are much higher than those of the planar grating-based directional beaming devices.Finally,we numerically demonstrated that the directional beaming scheme based on gradient metasurfaces can also be used to realize the directional beaming of the light waves emitted from the subwavelength aperture at the optical frequencies.(3)In the study of graphene-based dynamic surface plasmon(SPP)couplers,this thesis proposes graphene-based dynamic SPP couplers to solve the problem that both traditional prism/grating-based SPP couplers and metasurface-based SPP couplers are unable to be dynamically modulated.Here,each graphene unit can act as a scatterer to diffract SPP into the free-space propagation wave(PW)and add an additional scattering phase caused by graphene resonance on the PW,which can be modulated by changing the chemical potential of the graphene.Therefore,the phase distributions of the outgoing PW can thus be dynamically adjusted by modulating the chemical potential of each graphene unit.In-plane/out-of-plane SPP-PW conversions are demonstrated with graphene ribbon/block metasurfaces.Converting SPP to single-or two-beam PWs with variable radiation angles is realizable by varying the chemical potential of each graphene ribbon/block without re-optimizing the structural parameters.In addition,we have demonstrated that the conversion efficiency of SPP-PW can also be dynamically regulated by means of the chemical potential modulation.(4)In the study of graphene-based polarization switchable wavefront manipulation devices,this thesis proposes graphene-based polarization switchable wavefront control devices to solve the problem of available metasurfaces lacking the ability to dynamically switch the operating polarization states of the devices.First,we designed a novel graphene metasurface that can independently control the reflection phase of x-and y-polarized EWs.Then,the same wavefront is loaded on the x-and y-polarized EWs under different graphene chemical potentials,respectively,by using this graphene metasurface.Therefore,when we change the graphene chemical potentials,the operating polarization states of the devices will be switched correspondingly.Using this design principle,we designed and demonstrated a polarization-switchable beam deflector,focusing lens,and Airy beam generator in terahertz frequencies.We believe these devices have great application potentials in polarization multiplexing systems,such as dynamic polarization routing.