Nonlinear Metasurface Based On Optical Nanocavity

In recent years,optical metamaterials have attracted widespread attention as a new type of artificial materials.Optical metamaterials are still composed of traditional materials,but its effective permittivity and permeability are controlled by the geometric shape of the subwavelength artificial unitcells(or “meta-atoms”).Thus,optical metamaterials can exhibit the electromagnetic response beyond the traditional materials.Metamaterials provide new ideas and implementation methods for the design of optical devices,but there are still some problems prevent them from practical from applications.For example,the complexity of the meta-atoms is limited by nano-fabrication technology,and there are high loss at the wavelength of visible light.For solving these problems,ultra-thin optical metasurfaces are created.The optical metasurfaces inherit the advantages of the metamaterials composed of subwavelength meta-atoms.With only a layer of meta-atoms,metasurfacees can flexibly and freely adjust the properties of light,such as the amplitude,phase,polarization,spin and orbital angular momentum.The optical metasurface is simple in structure,intuitive in design,easy for fabrication,and benefit for integration,which greatly promotes the development of novel optical devices.In the field of linear optics,various functions such as optical focusing,holographic imaging,and beam shaping have been realized on the platform of metasurfaces.Metasurfaces have also received extensive attention in the field of nonlinear optics.For its ultrathiness,nonlinear optical metasurfaces are free of harsh phase-matching conditions.Meanwhile,we can control every meta-atoms to control the generated nonlinear light's phase,amplitude and polarization,and realizing various optical functions.In the field of nonlinear optical metasurfaces,how to improve the nonlinear conversion efficiency within the limited thickness of the metasurface has been the focus of related research.This thesis provides a nonlinear plasmonic metasurface composed with subwavelength-scale nano-cavities,for ehancing the second harmonic generation at the wavelength of near infrared.Also,the period of metasurface is adjusted for tailoring the electromagnetic environment at the frquency of the second harmonic,and the nonlinear conversion efficiency can be tailored.The nonlinear plasmonic metasurface proposed in this paper is composed of nanocavities,which is composed of a three-layer structure of a subwavelength-scale gold splitting-resonant ring,a silicon dioxide spacer layer,and a gold reflective surface.The gold splitting-resonant-ring is a meta-atom with excellent property in the field of the second harmonic generation.By forming a nanocavity with silicon dioxide and a gold reflective surface,the ability of energy localization and field enhancement of the nanocavity will significantly improve the nonlinear conversion efficiency of the metasurface.This thesis designs the nanocavities with the theory of cavities,and then uses the commercial simulation software COMSOL Multiphysics based on the finite element method for studying the linear response of the above metasurface in the frequency domain mode.Combined with the nonlinear hydrodynamic model of electron gas,the second harmonic generation efficiency of the metasurface also can be carried out and optimized by numerical simulation.The simulation results show that the metasurface consisting of the nano-cavities can improve the second harmonic generation efficiency by about 13 times compared to the metasurface consisting of only a single layer of splitting-resonant ring.Then,the metasurface samples were prepared by advanced nano-fabrication techniques and well characterized in morphology,and the preparation process and characterization methods were introduced in detail.Finally,the linear and nonlinear optical characteristics of the metasurface sample were measured experimentally.The reflectance spectrum at the wavelength of fundamental wave and the second harmonic generation efficiency were measured.It has been proved that the changing of period can change the electromagnetic environment at the frequency of the second harmonic,and the conversion efficiency of the second harmonic can be significantly tailored.Also,a high effective second order susceptibility has been demostrated in this thesis.And nanocavity metasurface provides a practical method for improving the efficiency of second harmonic generation,and has potential value in integrating nonlinear optical devices.