In this paper, we proposed a multilayer terahertz absorber composed of hybrid graphene and vanadium dioxide (VO₂). Based on electrical controlling of graphene and thermal tuning of VO₂, three different switchable absorption states are achieved in one structure. When VO₂ is in the metal phase and the Fermi level of graphene is set as 0eV, high-frequency broadband (bandwidth, 5.45THz) absorption from 4.5 to 9.95THz is demonstrated. While VO₂ is switched to the insulator state, absorption states depend on the Fermi energy of graphene. As the Fermi level changes from 1eV to 0eV, the absorption can be switched from low-frequency broadband (bandwidth, 2.86THz) to dual-frequency absorption. The effect of geometric parameters and fabrication tolerance on the robustness of the absorption properties is explored. The proposed absorber has three switchable states through modulation of graphene and VO₂, which is expected to realize potential applications in modulating, filtering, detecting, and other fields.

Recently, high-order topological photonic crystals (PhCs) have attracted huge research attention due to their novel physics mechanism and the application potential in integrated photonics. Based on the two-dimensional Su-Schrieffer-Heeger model, we construct and study the mutual coupling between the high-order corner states in 2D dielectric PhCs. Simulation results show that the Q-factor of such corner-localized quasi-bound states in the continuum (BICs) could be enhanced following mutual coupling in finite size. Furthermore, we study the side-coupled structure based on defect-hybrid waveguides and the edge state microring, the quasi-BIC microcavity. The refractive index sensing application based on corner-localized quasi-BICs shows outstanding simulated sensitivity (312.8 nm/RIU) and figure of merit (∼10³ 1/RIU). The robustness against fabrication errors due to its topologically protected nature makes it competitive compared with other quasi-BICs sensors.

A switchable multi-function terahertz metasurface employing phase transition material vanadium dioxide (VO₂) is presented and investigated. By varying the temperature, the hybrid structure can switch between transmission, absorption, and reflection modes in broadband ranges. When the temperature is below 340 K and VO₂ is in the insulating state, perfect polarization conversion is demonstrated. Efficient asymmetric transmission (AT) exceeding 0.7 is simultaneously achieved with an ultra-wide bandwidth of 3.6THz. When VO₂ is in the metal phase, it shows different modulation characteristics for x and y-polarized waves. The structure can absorb over 90% of y-polarized waves from 3.56 THz to 7.2 THz (bandwidth, 3.64 THz), while 85% of x-polarized waves are reflected from 1 THz to 9 THz (bandwidth, 8 THz). Compared with other related published works, the designed structure makes significant progress in integrated functionalities, operating bandwidth, and working efficiency. It shows great potential for use in terahertz dynamic control and multifunctional integrated systems.