1. Introduction

The absorption spectral response of conventional broadband metamaterial absorbers (MMA) is usually sensitive to the light polarization when oblique incident angle increases. Normally thickness is increased, sometimes much larger than operating wavelength, to avoid the exacebation of absorption and bandwidth under oblique incidence. We adopt the concept of uni-axial perfect match layer (UPML). Transform optics is reallized to analyze the relative permittivity and permeability tensors for a uni-axial medium to build a MMA which can achieve perfect absorption without extra thickness.

This proposed wide-angle, wideband, polarization-insensitive terahertz absorptive MMA consists of a paired slot-frequency selective surface (FSS), a vertical rod VIA and a split-ring resonator (SRR) based on the design mechanism of UPML. By tailoring the dispersion of periodic VIAs and SRRs, the components of our MMA satisfy the required macroscopic material anisotropy.

2. Theory and Structure

Fig. 1 illustrates the unit cell structure of our terahertz MM absorber consisting of a multi-layered structure with four components. The top layer is resistive chromium SRR. Details of the structure can be found in [1].

It is well known that the electromagnetic property of a uni-axial anisotropic medium can be described as an diagonal tensor. The relative permittivity (permeability) tensor for the uniaxial medium can be expressed as,

The SRR and VIA shown in Fig. 1 determine the optical axis directions of permittivity ε_a and permeability μ_c, respectively. Through derivation of Fresnel’s coefficient on the interface where the optical-axis is parallel to the normal vector of interface between isotropic medium and uniaxial medium, the reflectionless condition can be obtain as μ_d = ε_b, ε_a = ε_b^-1, and ε_a=μ_c [1], [2].

Fig. 2 (a) The real part of εa, and εb-1. (b) The real part of εa, μc and μd. The green dashed line indicates the permeability μd in vacuum.

Each component involving VIA-rod, SRR, and slot-FSS of MMA can be tailored to make macroscopic dispersion of each part satisfying the required UPML relation. The values of effective ε_a, ε_b^-1, and μ_c are closed to each other after 3 THz. Due to the lack of magnetic materials in high frequency regime, the value of μ_d for our proposed absorber is close to one without dispersion as shown in Fig. 2.

3. Full-wave Simulation

Fig. 3 Reflectance spectra of the proposed structure as function of incident angle for (a) TE polarization and (b) TM polarization. The white dash curves and black dash-dot curves indicate the gradient contours for 10% and 50% reflectance, respectively.

The MMA shown in Fig. 1 is simulated and optimized using the commercial full-wave simulation software (Ansys HFSS). The simulated reflectance spectra (Fig. 2(a) and Fig. 2(b)) show a outstand performance. The reflectance of the MM absorber is below 10% in a wide incident angles ranging from 0^o to 60^o for both TE and TM polarizations within the frequency regime of 0.9 to 10.5 THz, corresponding to 168% bandwidth to the central wavelength.

4. Summary

We demonstrated a wideband and wide-angle terahertz absorber based on the concept of UPML. The SRR, a bi-layered slot-FSS, and VIA structures were optimized to achieve equivalent electric and magnetic plasma frequencies and similar dispersion profiles to approach the effective permittivity and permeability tensor of an ideal UPML. The reflectance can achieve excellently below 10% from 0.9THz to 10.5THz, corresponding to 168% of the central frequency under 0^o to 60^o incidence, for both TE and TM waves.