We demonstrated a dynamically controlled broadband terahertz (THz) metamaterials absorber, which composed of continuous vanadium dioxide (VO₂) film, a silicon dioxide (SiO₂) layer, and a structured borophene layer. When VO₂ is in its metallic state and the armchair direction of borophene along x axis, the proposed absorber realizes an absorptivity peak value of 100% at 7.2 THz for y polarized normal incidence, and an absorptivity peak value of 79% at 8.9 THz for x polarized normal incidence. It is the anisotropic property of borophene that results in the absorptivity difference for x and y polarization in the whole frequency range. Simulated electric field distribution and surface current oscillation has been extracted to explain the physical mechanism of THz wave absorption. Through modifying the geometric parameters of metamaterials microstructure, the broadband absorption performance can be tailored passively. Additionally, the proposed metamaterials absorber has been actively controlled by manipulating the carrier density of borophene and the conductivity of VO₂, respectively. The absorptivity can be switched from 45% to 100% at 7.2 THz by changing the carrier density of borophene, and from 22% to 100% at 7.2 THz by changing the conductivity of VO₂. Moreover, the proposed absorber exhibits an excellent operation tolerance for oblique TE and TE polarized incidence from 0° to 60°. This work provides a novel approach to design dynamically controlled broadband THz absorbers, which reveals promising applications in the devices of optoelectronic switches, cloakings, filters, and sensors, etc.

Artificially designed modulators that enable a wealth of freedom in manipulating the terahertz (THz) waves at will are an essential component in THz sources and their widespread applications. Dynamically controlled metasurfaces, being multifunctional, ultrafast, integrable, broadband, high contrasting, and scalable on the operating wavelength, are critical in developing state-of-the-art THz modulators. Recently, external stimuli-triggered THz metasurfaces integrated with functional media have been extensively explored. The vanadium dioxide (VO₂)-based hybrid metasurfaces, as a unique path toward active meta-devices, feature an insulator–metal phase transition under the excitation of heat, electricity, and light, etc. During the phase transition, the optical and electrical properties of the VO₂ film undergo a massive modification with either a boosted or dropped conductivity by more than four orders of magnitude. Being benefited from the phase transition effect, the electromagnetic response of the VO₂-based metasufaces can be actively controlled by applying external excitation. In this review, we present recent advances in dynamically controlled THz metasurfaces exploiting the VO₂ phase transition categorized according to the external stimuli. THz time-domain spectroscopy is introduced as an indispensable platform in the studies of functional VO₂ films. In each type of external excitation, four design strategies are employed to realize external stimuli-triggered VO₂-based THz metasurfaces, including switching the transreflective operation mode, controlling the dielectric environment of metallic microstructures, tailoring the equivalent resonant microstructures, and modifying the electromagnetic properties of the VO₂ unit cells. The microstructures’ design and electromagnetic responses of the resulting active metasurfaces have been systematically demonstrated, with a particular focus on the critical role of the VO₂ films in the dynamic modulation processes.

The polarization topology around the bound states in continuum (BIC) affects the optical vortex (OV) beam generation. We propose a cross-cross-shaped resonator based on a THz metasurface to realize an OV beam generator in real space by exploiting the inherent winding topology around the BIC. The BIC merging at the point Γ is achieved by tuning the width of the cross resonator, which significantly improves the Q factor and enhances the field localization. Furthermore, the switching between the high-order OV beam generator governed by the merged BIC and the low-order OV beam generator is realized. This extends the application of BIC in modulating orbital angular momentum.

Spatiotemporal optical vortices (STOVs) exhibit characteristics of transverse orbital angular momentum (OAM) that is perpendicular to the direction of pulse propagation, indicating significant potential for diverse applications. In this study, we employ vanadium dioxide and photonic crystal plates to design tunable transreflective dual-channel terahertz (THz) spatiotemporal vortex generation devices that possess multipolarization adaptability. In the reflection channel, we achieve active tunability of the topological dark lines by utilizing circularly polarized light, based on the topological dark phenomenon, and observe variations in the number of singularities across the parameter space from different observational perspectives. In the transmission channel, we generate independent vortex singularities using linearly polarized light. This multifunctional terahertz device offers a novel approach for the generation and active tuning of spatiotemporal vortices.