Terahertz (THz) is a electromagnetic wave with the frequency range of 0.1‒10 THz, and is gradually playing an important role in many fields. However, because traditional electronic and optical design methods consider the adjacent microwave and infrared optical bands, the application of the THz band is not easy to directly expand, which will undoubtedly greatly hinder further development of THz technology. Thus, there is an urgent need for new THz device design methods to solve this difficulty. Metamaterials are composed of a series of micro- and nanostructures with artificially designed periodic arrangements, whose size, shape, and distribution can produce optical responses that natural materials do not exhibit after careful design. Two-dimensional metamaterials, i.e., metasurfaces, with a simple process flow and low processing cost, have gradually replaced metamaterials in recent years and have become a popular research topic. The application of metasurfaces to THz technology overcomes the limitations of traditional materials, contributing to their development. With the increasing demand for corresponding applications, researchers have shifted their attention from single passive hypersurfaces to tunable active metasurfaces. These tunable metasurfaces are often dependent on several tunable materials. In particular, in the THz band, vanadium dioxide (VO₂) is an excellent tunable material that is being actively investigated by researchers due to its abrupt change in conductivity of four to five orders of magnitude before and after the phase transition temperature, which allows it to complete the insulating to the metallic phase transition.

A metasurface comprising a periodic array of double-gap split-ring resonators, with VO₂ structures embedded in the gaps, was considered in this study. The spectral responses to different VO₂ conductivities and electric field distribution images of the corresponding modes were first simulated using the commercial simulation software, CST. Next, samples were obtained via conventional lithography and other micro- and nano-processing techniques, which were then characterized experimentally using THz time-domain spectroscopy (TDS). First, the samples were heated directly using a hot stage, followed by laser pumping, strong THz pumping, and THz detection for mode characterization.

The simulation results clearly show that with increasing VO₂ conductivity from 10 to 2×10⁵ S/m, the resonant frequency red-shifts from 0.75 to approximately 0.5 THz, the gap of the metal arm is approximately filled, and the whole structure completes the transition from mode 1 to mode 2. Field monitoring shows that before the phase transition, mode 1 is a magnetic dipole resonance with an enhanced electric field at the opening; whereas, after the phase transition, mode 2 is an electric dipole resonance after the conduction of the metal arm, and its electric field is mainly distributed in the upper and lower metal arm regions (Fig. 1). Direct characterization of the sample heating confirmed the simulation results. At temperatures lower than 57 ℃, the resonant frequency of the structure remained at approximately 0.7 THz, indicating that the temperature change at this time could not substantially affect the conductivity of VO₂, and the sample was in the mode 1 state. As the temperature increases further, the resonant frequency gradually red-shifts to 0.45 THz, accompanied by a gradual decrease in the amplitude of the resonant peak, reaching a minimum of approximately 0.4 at ~64 ℃. With continuing increase in temperature, the resonant frequency continues to red-shift, and the amplitude becomes larger, indicating that VO₂ is transitioning between the insulating phase and the metallic phase. For temperatures higher than 73 ℃, the resonance mode does not change significantly in both amplitude and resonance frequency, and tends to stabilize, at which time the VO₂ conductivity tends to saturate and completes the filling of the metal arm gap, indicating that the metasurface is in the state of mode 2. Therefore, by directly heating the sample, a conductive channel at the gap is successfully constructed, the transition from resonant mode 1 to mode 2 of the metasurface is completed, and the variation in the resonant frequency with temperature provides a more direct reflection of the mode switching (Fig.2). Laser pumping requires heating the respective sample to near the phase transition temperature; similarly, different laser powers can induce a VO₂ phase transition (Fig.3). Finally, strong THz pumping of samples with different intensities can also produce the VO₂ phase change. It is worth noting that although VO₂ is in the vicinity of the phase transition temperature, broadband modulation, such as temperature and laser pumping excitation, cannot be fully achieved under strong THz field excitation (Figs.5 and 6).

In this paper, a tunable, embedded VO₂ hybrid metasurface is proposed to realize the dynamic switching of resonant modes from a high frequency of around 0.7 THz to a low frequency of around 0.43 THz in the THz band. The VO₂ at the opening of the resonant ring undergoes a sudden change in conductivity by more than four orders of magnitude before and after the phase transition, constructing a conductive channel in the metal arm of the resonant ring and thus completing mode switching. The feasibility of this mode-switching was verified experimentally through various applications of thermal, laser, and strong-field THz excitations. Although the physical mechanisms of the former and latter two differ, the multimode dynamic excitation manipulation of THz waves presents a feasible idea for practical applications.