| Terahertz absorbers have important application background in terahertz capture,detection and sensing.The absorber based on traditional materials has the disadvantages of low absorption efficiency,narrow band and large volume,which limits the development of terahertz absorbers.By optimizing the design of lightweight,ultra-thin metamaterials,broadband perfect absorption can be achieved in terahertz region.However,it is difficult to achieve continuous(quasi-continuous)resonance state in terahertz absorbers designed with geometric patterned graphene,and the ability to expand absorption bandwidth is limited.In this paper,the graphene metasurface with modulated conductance is studied,which provides a simple and easy-to-obtain conductive modulation function for the technical design.It avoids the problem that the complex conformal transformation function is difficult to determine in the process of technical application,and provides a new idea for the development of terahertz ultra-broadband perfect absorption devices.Based on the conformal transformation method,a simple sinusoidal conductivity modulation function is proposed.The calculation of terahertz transmission spectrum and absorption spectrum shows that graphene metasurface with modulated conductivity can stimulate a series of surface plasmon modes.The electric field enhancement and electric field constraint appear at the position of minimum conductivity.We analyze the influence of conductance modulation depth and modulation function on resonant mode density,and discussed the influence of modulation period on resonance frequency.By increasing the modulation depth,higher resonance modes can be excited,leading to larger resonance mode density.The perfect terahertz ultra-broadband absorption characteristics of the single-layer graphene metasurface with modulated conductivity were studied,and the effects of air and substrate environment on the terahertz spectral response were discussed.The terahertz absorption characteristics of the metal reflection enhanced metasurface system were analyzed.We analyzed the effects of graphene carrier mobility and dielectric spacer thickness on resonant half-width,absorption rate and absorption bandwidth.By optimizing the perfect terahertz ultra-broadband absorption characteristics,the monolayer metasurface system can achieve continuous(quasi-continuous)resonance state.The ultra-broadband near-perfect absorption can reach 3.71 THz,which covers more than one third of the THz region,with absorption efficiency of more than 90%.The terahertz ultra-broadband absorption characteristics of grating reflection enhancement are analyzed.Bragg grating coupled with single-layer metasurface structure can achieve 1.38 THz broadband absorption,with absorption rate of more than 80%.Terahertz ultra-broadband perfect absorption characteristics of bilayer graphene metasurface with modulated conductivity are theoretically investigated.We analyzed the THz resonance spectra of the double-layer inverse phase and double-layer in-phase conductivity modulated graphene system,and the influence of the near-field mode coupling effect on the resonance spectral characteristics and resonance mode density.By enhancing the near-field mode coupling effect,the double-layer graphene system modulated by in-phase conductivity can excite higher-order surface plasmon modes.By optimizing the parameters of the ultra-broadband absorption performance of the double-layer in-phase conductivity modulation graphene system,the continuous resonance state is realized,and the near-perfect absorption is realized in the terahertz ultra-wideband at 6.35 THz with absorption rate of more than 80%,which covers almost two thirds of the terahertz region.The designed graphene conductance modulated terahertz absorber has the advantages of ultra-broadband absorption and more than80% theoretical absorption efficiency,which is of great significance to promote the research of broadband terahertz absorber. |
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