Objective Terahertz (THz) absorbers are essential devices for suppressing electromagnetic interference or pollution in related THz system and play an increasingly important role in THz applications such as wireless communications, imaging, and sensing. Metamaterials, composed of artificially designed periodic subwavelength structure arrays, have the desired electromagnetic response beyond traditional materials. Since Landy et al. proposed the classical metamaterial- insulator-metal (MIM) absorber configuration, various THz absorbers integrating metamaterial and functional materials have been developed in both fundamental and applied research. Among them, graphene-based hybrid metamaterial THz absorbers have attracted wide attention due to their tunable transmission and flexible design. However, the natural optical properties of the ultrathin 2D graphene, like low absorption of incident waves and weak electromagnetic response, limit its applications in high-performance THz absorbers. The emerging 3D Dirac semimetal (DSM) exhibits high carrier mobility and tunable Fermi level comparable to graphene and overcomes the above-mentioned inherent disadvantages of graphene. In this study, a tunable four-band THz metamaterial absorber based on Dirac semimetal is designed. Using the finite integration method, the absorption characteristics in THz regime are systematically simulated in response to the structural parameters and Fermi levels of DSM. Methods We introduce a dynamically tunable four-band THz metamaterial perfect absorber based on a three-dimensional DSM. The designed MIM structure consists of DSM metamaterial layer, a dielectric layer, and a metal substrate layer from top to bottom (Fig. 1). First, we calculate the Fermi level-dependent complex permittivity of 3D DSM in the frequency range of 3.0 THz to 5.2 THz based on the Kubo formula and the two-band model (Fig. 2). Then, we perform finite-integration-method-based simulations to reveal electromagnetic field distribution and absorption spectra, and we simulate the absorption spectra of the two substructures of the rectangular ring and the circle separately. In addition, we discuss the effects of structural parameters and Fermi levels on spectral evolution and absorption properties. Results and Discussions From the electromagnetic field distribution, we can see that the strong magnetic resonance is mainly generated in the PI medium layer, while the electric resonance is dominantly excited on the surface of the DSM layer (Fig. 4). Together, they give rise to the four-band near-perfect absorption of THz waves, and this can also be interpreted by the three equivalent sub-structure resonators (Fig. 5). The absorption intensity and frequency remain stable when changing the structural parameters of R, W1, L1, and h2 within a small range (Fig. 6). As the Fermi level of DSM increases from 50 meV to 75 meV, the frequencies of the resonant peaks M1-M4 show a blue-shift trend, and the corresponding frequency shifts are 110.5, 99.6, 45.0, and 85.5 GHz, respectively (Fig. 7). Meanwhile, the absorption intensities are relatively high with a value above 95%. By tuning the surrounding refractive index (RI) from 1.00 to 1.16, a maximum sensitivity up to 721.8 GHz center dot RIU-1 can be obtained (Fig. 8). Conclusions We demonstrate a tunable THz metamaterial absorber based on 3D DSM through numerical simulations. The simulated electromagnetic field distributions and the impedance-matching analysis at the resonant frequency suggest that the perfect multi-band absorption is induced by the electric resonance excited on the DSM layer and the strong magnetic resonance formed in the PI layer. The resonant frequency and absorptivity show good tunability as the Fermi level of DSM changes from 50 meV to 75 meV. There is a linear relation between the resonant frequency and the surrounding refractive index, leading to a large sensitivity of 721.8 GHz center dot RIU-1 in the RI sensing range of 1.00 to 1.16. Furthermore, the absorption properties are proved to be insensitive to the polarization angle of incident THz wave. These findings not only provide an important reference for the development of 3D DSM based absorbers but also pave the way for the potential applications of tunable and broadband THz devices.
