Resonance Mode And Polarization Conversion Properties Of All-Dielectric Metasurface

Metasurfaces made of periodic artificial microstructures exhibit many novel properties that natural materials do not have,which are of great importance for both fundamental studies and the development of functional optical devices.Due to intrinsic ohmic loss of metals in mid-infrared spectral region,metallic metamaterials are often limited in terms of spectral linewidth and energy efficiency.As some dielectric materials exhibit negnigible optical loss in the mid-infrared,all-dielectric metasurfaces present a pathway in studying new metamaterial properties and functional devices.In this thesis,we study resonance mode and polarization properties of all-dielectric metasurfaces using amorphous silicon.We design and fabricate metasurfaces made of periodic silicon cuboids,and demonstrate cross-polarization conversion based on anistropic excitation of electric diople and magnetic dipole resonances.Our main research results are as follows:1.All-dielectric metasurfaces composed of periodic silicon cuboids on CaF2 substrate are designed and their resonance modes and polarization conversion properties are analyzed at different polarization conditions.Our simulation results show that there exist magnetic resonance mode and high-order electric resonance mode at the wavelengths of 11.47 ?m and 13.33 ?m respectively for incident x-polarization.For incident y-polarization,electric dipole resonance is excited at the wavelength of 11.61?m.In addition,when the incident polarization angle is 45°,cross polarization conversion occurs at the wavelength of 11.55 ?m.The cross-polarized reflection coefficient Rcross is as high as 87.77%,while the co-polarized reflection coefficient Rco is only 8.13%,leading to a polarization conversion efficiency PCR of 91.52%.By optimizing geometric parameters of the cuboid silicon,Rcross up to 96.6%at the wavelength of 10.91 ?m is obtainable in theory.2.On basis of the theoretical results,metasurface samples of silicon cuboids are fabricated,and their polarization conversion properties are verified in experiments.Our infrared spectral measurements show that magnetic resonance is excited at about 12 ?m wavelength for x-polorization incident light at 15° incidence angle.As the incident light changes to y-polarization,electric resonance at about 12 ?m is excited.When the polarization angle becomes to be 45°,cross-polarized reflection coefficient Rcross is measured to be 19.83%,exhibiting an obvious polarization conversion character.The discrepancies between experiments and simulation results in terms of resonant wavelengths and polarization conversion efficiency are mainly caused by the size error induced during micro-fabrication processes.