Bi-tunable terahertz absorber based on strontium titanate and Dirac semimetal.

We proposed a polarization-insensitive absorber based on strontium titanate (STO) and bulk Dirac semimetal (BDS) in the terahertz (THz) region. The center frequency of the absorption peak can be independently regulated by temperature or Fermi energy level of STO or BDS, respectively. The numerical simulation result reveals that the peak absorptivity reaches to 99.98% at 2.16 THz when the temperature and Fermi energy were set at 300 K and 20 meV, respectively. Interestingly, by adjusting the temperature of STO from 250 to 400 K, the simulation results indicate that the center frequency can be tuned from 1.94 to 2.53 THz, and peak absorptivity can be maintained above 99% at normal incident. As the Fermi energy EF of Dirac semimetal increases from 10 to 60 meV, the center frequency can be changed from 2.14 to 2.44 THz and the amplitude of absorption peaks can be tuned from 99.9% to 82.8%. Impedance matching theory was used to understand the tunable performance. Furthermore, interference theory was employed to further explain the absorption mechanism of the proposed absorber. The absorber achieves bi-controlled absorptance via two independently controllable methods, which may provide guidance to research tunable, smart and multifunctional terahertz devices.