With the continuous advancement of terahertz technology, its applications in wireless communication, medical imaging, and security screening are expanding. Metasurfaces are widely used in terahertz device designs due to their effective modulation properties for terahertz waves. Traditional metallic materials fail to achieve active tuning of terahertz devices. Therefore, the design of terahertz devices with multifunctionality using tunable materials such as vanadium dioxide and Dirac semimetals has become increasingly attractive. In this work, we propose a metasurface with configurable functions based on Dirac semi-metallic mirror-symmetric double-opening rings. By utilizing the phase transition property of the vanadium dioxide, we have realized the switching of the filter and sensor functions in a single device structure. The results not only demonstrate the possibility of implementing the multifunctional metasurface design in the terahertz band but also can promote the application of terahertz technology in the future.

In this study, a metasurface terahertz device with switchable sensor and filter functions was designed by utilizing the Dirac semimetal and vanadium dioxide. When the vanadium dioxide was transformed from the insulating state to the metallic state, the structure could operate as a sensor and a filter, respectively. By using the frequency-domain finite-element method (FEM) based on the commercial software CST Microwave Studio, the performance of the designed device was simulated. The transmission spectra in both functional modes were studied. Based on the three-dimensional electromagnetic simulations, the analysis of the physical mechanism of the device was carried out through the resonance characteristics and the electric field distributions. Moreover, the sensitivity of the device used as a sensor was investigated through simulation by changing the samples with different refractive indices.

By changing the ambient temperature, the vanadium dioxide in the designed device can be transformed from the insulating state to the metallic state, so that the device works in sensor and filter mode, respectively. When the vanadium dioxide is in the insulating state, the device is in the sensor mode, and it achieves sharp transmission dips (Fig. 3). We explain the resonance principle through the transmission spectrum and the electric field distributions (Figs. 4-6). In addition, the resonance can be enhanced as the parameter d increases (Fig. 7), simultaneously causing a change in the Q value at each resonance point. As d increases, the Q value of p3 increases, while other resonance points decrease (Fig. 8). Moreover, the Fermi energy level also influences the resonant frequency and resonant intensity of the sensor (Fig. 9). Simulations show that the sensitivity can be increased with the increase in the sample thickness. When the sample thickness is 10 μm, the sensitivity is 106 GHz/RIU, and when the sample thickness is increased to 45 μm, the sensitivity reaches a saturation value of 226 GHz/RIU (Fig. 14). Moreover, the effect of the distance between the sample and the metasurface on the sensitivity was explored when the sample thickness was fixed at 10 μm. The results show that as the distance increases, the sensitivity increases and reaches a maximum value of 130 GHz/RIU (Fig. 15). When the vanadium dioxide is in the metallic state, the device turns out into the bandpass filter mode. It operates with the insertion loss of 1.3 dB at the center frequency of 0.84 THz while the return loss is 12.7 dB (Fig. 16). The resonance mechanism of the filter was discussed through the transmission curves and electric field distributions. The top vanadium dioxide layer can prevent the terahertz waves from entering the metasurface, thereby suppressing the electric field intensity of the resonance rings (Figs. 17-19). Furthermore, the center frequency can be tuned by adjusting the length of the parameter d (Fig. 20).

A terahertz metasurface based on vanadium dioxide and Dirac semimetal is proposed, which can switch between two functions by adjusting the conductivity of vanadium dioxide. When vanadium dioxide is in the insulating state, the metasurface is a terahertz sensor, and the sensitivity of the sensor is related to the thickness of the sample and the distance between the sample and the sensor. When the vanadium dioxide is in the metallic state, the metasurface is a terahertz bandpass filter, which has a center frequency of 0.84 THz. The insertion loss and return loss at the center frequency are 1.3 dB and 12.7 dB, respectively. The resonance principle is investigated by analyzing the electric field distributions of this metasurface. This work demonstrates the possibility of implementing the multifunctional metasurface design in the terahertz band. The structure proposed in this paper has potential applications in future terahertz sensor, filter, and multifunctional device designs.