Ultrathin Terahertz Triple-Band Metamaterial Absorbers: Consideration of Interlayer Coupling

We present a polarization-insensitive air-spaced triple-band metamaterial perfect absorber (MMPA), consisting of a metamaterial layer and metallic ground plane operating at terahertz frequencies. Three near -unity absorption peaks can be individually determined by the geometry of the ring resonators within one unit cell, since the inter-unit-cell coupling is negligible. However, for sufficiently small interlayer spacing (< 20 mu m), coupling between the metamaterial layer and the ground plane is non-negligible. Therefore, near-field interactions must be taken into account for a full understanding of the electromagnetic response (EMR). Interference theory is often used to model the EMR of MMPAs analytically, in which interlayer coupling between the metamaterial and ground plane is usually neglected, resulting in a predicted blueshift of the absorption peaks in comparison to experiment. To account for near-field coupling, we incorporate correction terms into the analytical interference model by taking into account the effective interlayer capacitance and inductance. This results in good agreement between interference theory and experiment (and full-wave numerical simulations). Our findings demonstrate that interlayer coupling is an important design parameter for ultrathin MMPAs.