A metal-free polarization-insensitive ultrathin and ultra-wideband terahertz absorber with wide incident angle

In this work, a novel metal-free absorber structure design is proposed with an ultra-thin geometry and ultrawideband response in the lower terahertz region. The unit cell of the proposed absorber is a simple tri-layered structure with silicon as a substrate sandwiched between the silicon resonator layer at the top and a graphite layer at the bottom. The absorber displays a near-perfect absorption for a wider range of frequency spectrum between 2.7 THz to 9.2 THz covering a bandwidth of 6.5 THz and a relative bandwidth of 102%. The proposed absorber exhibits a multi-band response in close proximity when coupled by a mode-hybridization technique resulting in an ultra-wide-band response. The interaction between the resonating modes of the silicon disc resonator layer and multiple modes in the dielectric cavity is responsible for this ultra-wideband absorption response. The performance of the proposed absorber is analyzed and optimized for various geometrical parameters. Also, the frequency response tunability behavior is reported in contrast to certain geometrical parameter variations. The simulation results are validated using the equivalent circuit modeling technique. The absorber response is polarisation insensitive owing to its symmetrical structure and is angularly stable for a wider angle of incidence up to 70 degrees. The resonator-free bi-layered structure of the proposed absorber can be utilized as a narrowband narrow-scale sensor. The sensing performance of the proposed absorber is analysed across a wide frequency range which makes it a suitable candidate for bio-sensing applications.