Design of tunable metasurfaces for ultra-broadband absorption in terahertz imaging applications

Terahertz metasurface absorbers with broadband and tunable response are significant for security, sensing, and imaging applications. Currently, most of the reported works in the literature have lower bandwidths, if not have complex designs, and are not actively tunable. In this article, we design a tunable metasurface absorber with an average absorption of 96.5% from 0.59 to 2.23 THz, giving a bandwidth of 1.64 THz under normal incidence of TE polarized plane wave. Our design comprises a U-shaped cavity based metasurface pattern made of silicon, coupled with a metallic ground plane. The ultra-broadband absorption is attributed to the superposition of multiple hybrid electric and magnetic resonant modes and transmission blocking by the ground plane. The simulation results are supported by the impedance matching theory for the metasurface absorber. Furthermore, a fabrication methodology and tolerance to the fabrication errors have been discussed for the practical realization of the proposed design. Our design shows wide-angle stability (up to 50 degrees), which makes it suitable for thermal imaging applications. This tunable absorber can also work as a narrow band absorber upon optical excitation of silicon. The figure-of-merit calculation and operating bandwidth suggest that our absorber can outperform recently reported silicon based broadband absorbers.