| Subwavelength optical structures have broad application prospects in the fields of biochemical sensing,nano-lithography,and high-resolution microscopy due to their optical properties such as breaking the diffraction limit,near-field local enhancement,selective absorption and radiation.With the micro-optics theory improvement and related fabrication technology development,the research of micro-nano optical device has gradually become one of the important topics in contemporary optics and related regions.Among them,sub-wavelength optical perfect absorber can achieve near-perfect absorption of electromagnetic waves at specific wavelength or targeted spectral band,and has become a research hotspot in defense and civilian areas that can be utilized for stealth technology,photovoltaic systems,image processing,and high-sensitivity sensing in recent years.Theoretically combining optical film interference with surface plasmon resonance,in this thesis,we proposed a kind of bandwidth switchable bidirectional perfect absorber that constructed by sub-wavelength thin film system and optical grating structure.We analyzed the absorption characteristics of this absorber numerically,and investigated its' physical mechanism for achieving near-perfect absorption.Major research works are as follows:First,based on the fundamental principle of thin film interference,we designed a six-layer thin film structure with alternating metal and medium.The simulation results of the combination of finite difference time domain method and the transfer matrix method show that under the combined effect of the wavelength selective absorption of the asymmetric Fabry Perot cavity and the destructive effect of thin film interference,when the light source is incident on a surface covered with a thin metal film,the absorber can have a narrow-band absorption rate of 99.90% at a wavelength of 771 nm with a half-value width of 20 nm;When the light source is incident on the surface covered with the dielectric film,the absorber achieves an average broadband perfect absorption of 96.02% in the wavelength range of500-1450 nm.According to Kirchhoff's law,the absorbance of an absorber in a thermal equilibrium state is equal to its emissivity,so one side of the bidirectional absorber can be used for broad-spectrum absorption,and the other side can be used for narrow-band thermal radiation.Furthermore,we utilized finite difference time domain method and rigorous coupled wave analysis to design a bidirectional subwavelength one-dimensional grating absorber with alternating metal-medium arrangements.When the light is incident to the grating surface with a dielectric film on the top layer,the structure has an average absorption rate of 91.46% in the wavelength range of 500-3500 nm under the excitation and coupling oflocalized surface pasmon resonance,cavity mode and guided mode;when the light is incident to the grating surface with a metal film on the top layer,due to the co-excitation of localized surface plasmon resonance and magnetic polariton,the structure has a narrow-band absorptivity of99.90% at 2793 nm with a half-peak width of 169 nm,and a refractive index sensing sensitivity of 1397nm/RIU.Finally,we summarized the whole work and evaluated the prospected application value of the proposed sub-wavelength tunable band bidirectional absorber in solar energy absorption/re-radiation and biosensing fields.We also predict our future work that includes fabricate the designed absorber device,further optimizing theabsorption characteristics of this absorber device by using other materials with dispersive propertiesandexperimental study of its thermal stability,as well as explore its perfect absorption in the ultraviolet band. |
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