A Theoretical and Experimental Investigation on Infrared Metamaterial Absorbers

Abstract

Metamaterials are artificially engineered structure with unique electromagnetic properties that can not be found in nature. They have many potential applications, and one of its most important applications is metamaterial absorbers. The designing of metamaterial absorbers is based on simultaneous ex citations of an electric dipole and a magnetic dipole resonances. Metamaterial absorber is typically a tri-layer structure with top metallic patterns structured at a sub-wavelength scales, a bottom metallic ground layer and a insulator layer in the middle. The top periodic structure functions as electric resonators driven by the electric field of the incident electromagnetic waves. The magnetic response of the structure is determined by the coupling of the two metallic layers and the dielectric layer. The metallic ground plane needs to be thicker than the skin depth to block any transmission. By altering the geometry sizes of the elements in the structure, the effective permittivity and permeability can be tuned to match the free space impedance, leading to a perfect absorption at certain wavelength. In the past few years, due to the demands of chemical detection and biological sensing, mid-infrared perfect metamaterial absorbers have been studied. For the broadband metamaterial absorber, we proposed a metal-dielectric-metal structure with top metallic patterns based on uniform raindrop shape. The absorption spectra and electromagnetic field distributions of the structure were numerically calculated by the finite element method based on commercial package COMSOL Multiphysics. Then we designed a braodband metamaterial absorber based on multiple sizes of raindrop shaped resonators. The fabrication of the proposed metamaterial absorbers was performed by E-beam lithography method. Following is the measurement of the absorption spectra using Fourier transform infrared spectrometer and the comparison with simulation results. Also, a five-band terahertz absorber with high absorbance was proposed and designed. The designed absorber is insensitive to both TE and TM polarization incident waves. The physical origins of the characteristics exhibited by this absorber can be attributed to dipolar and hexapolar resonances, as established by analyzing the electrical field density. Moreover, the influences of the main structural parameters and configurations on the absorption frequencies were studied. By varying several structural parameters, such as square ring length, dielectric thickness, and cross length, the absorption frequencies can be shifted to higher or lower values. In addition to the adjustment of absorption frequencies, the number of total resonance bands can also be adjusted by revising the structural configurations.