Research On Tunable Metamaterial Absorber Based On Fractal Structure

Metamaterial absorbers are artificially synthesized sub-wavelength periodic wave absorption structure,which has the characteristics of ultra-thin,ultra-broadband,high absorption and miniaturization.In addition,the terahertz,mid-long infrared and laser bands have broad application prospects in the fields of optical imaging,infrared detection,and military camouflage,respectively.Based on the characteristics of self-similarity,iterative and multi-scale of fractal structure,the related wave absorbing device designed in this paper has become a powerful tool for expanding frequency band and realizing high broadband absorption.Traditional metamaterial absorbers are fixed structure size and limited by specific optical properties after fabrication.After introducing some materials with tunable dielectric properties into the structure,it can meet the requirements of dynamically tunable absorbing without reconstructing the structure size.The design of the absorber in this paper is novel and has the characteristics of simple structure,strong function,flexible design and wide application.Based on this goal,this paper uses the finite element analysis software COMSOL Multiphysics to design three kinds of tunable metamaterial absorbers based on fractal structure.The following works are proposed in this paper:1.In the terahertz waves,a tunable broadband double-layer terahertz absorber based on cross-fractal graphene is designed.The one-to four-Level fractals resonators are integrated into a supercell,and the coupling and superposition of adjacent resonators form an ultra-broadband high absorber.After numerical simulation and optimization of the fractal structure,high absorption can be achieved in the range of 0.55-1.81 THz.（absorption>80%）,the center frequency f_c=1.18 THz,and the relative bandwidth is 106.8%,the designed terahertz absorber has the advantages of ultra-broadband,ultra-thin,flexible and tunable.By adjusting the bias voltage to change the Fermi level of graphene,the dynamic tuning of the terahertz absorber is realized.Compared with previous work,the model adopts fractal structure to extend the bandwidth,and graphene provides dynamically tunable properties with more degrees of freedom.2.In the infrared waves,a dual-band tunable broadband metamaterial absorber is designed,achieving high absorption in 3-5μm mid-wavelength infrared and 8-12μm long-wavelength infrared region.Numerical simulation shows that the designed single-layer 3-Level graphene-metal fractal cross absorber produces broadband absorption of 2.86μm（absorption>90%）in the long-wavelength infrared region.By adding the bottom layer 3×3graphene-metal fractal cross array structure,a resonant absorption peak is generated in the mid-infrared region,and the broadband absorption of 0.38μm（absorption>80%）,and it achieves broadband absorption of 2.48μm（absorption>90%）in the long-wavelength infrared region.The absorber achieves the absorption peaks of mid-wavelength infrared and long-wavelength infrared atmospheric windows.For the double-layer graphene-metal structure,the metal fractal cross layer mainly provides high absorption,and the graphene layer mainly plays a role in regulating resonance bandwidth and absorptivity.3.In the laser waves,a dynamic tunable dual-laser perfect absorber based on the combination of Helmholtz resonance and fractal structure is designed.The Helmholtz resonator is used to achieve accurate regulation at large frequency ratio,which is compatible with infrared camouflage and dual laser band stealth.The optical constants of Ge₂Sb₂Te₅ that change significantly before and after the phase transition are used to control its transition between amorphous and crystalline states,dynamically tunable the absorption rate of a specific band,and realize the tunable perfect absorption of dual laser bands.Numerical simulations show that under the third-order square fractal,the absorber can achieve ideal absorption in both laser bands（absorption>99%）when Ge₂Sb₂Te₅ is amorphous state,and can achieve wide-angle tunable laser stealth,it has good infrared camouflage compatibility in transparent atmospheric windows.