A multifunctional phase-change terahertz absorber enabling dynamic switching between dual ultra-high-Q narrowband absorption and ultra-broadband absorption.

Terahertz absorbers capable of dynamically switching between high-quality-factor (high-Q) narrowband response and broadband absorption are of great importance for multifunctional integrated photonic devices. This work proposes a hybrid metasurface driven by the vanadium dioxide (VO2) phase transition, achieving reconfigurable switching of electromagnetic resonance mechanisms through the insulator-to-metal transition. In the insulating state of VO2, structural asymmetry is introduced into the Si resonators, transforming the symmetry-protected bound states in the continuum (BICs) into dual quasi-bound states in the continuum (quasi-BICs). Multipole decomposition and electromagnetic field analysis reveal that the two quasi-BIC modes are governed by distinct radiative regulation mechanisms: quasi-BIC1 is dominated by the magnetic dipole (MD) with accompanying magnetic quadrupole (MQ) coupling, whereas quasi-BIC2 is dominated by the MQ with a contribution from the MD. Under these modes, the device achieves high quality factors of 5717.30 and 3636.40, respectively, and figure of merit (FOM) values exceeding 103 RIU-1. Upon the transition of VO2 to the metallic state, the system switches from the quasi-BIC radiation-suppressed mode to localized electric dipole (ED) resonance at the VO2-Si interface and, combined with a Fabry-Pérot cavity, realizes ultra-broadband absorption exceeding 95% over the 2.45-8.79 THz range. This work achieves dynamic reconfiguration between dual quasi-BIC high-Q sensing and broadband absorption mechanisms within a single metasurface, with a third resonance originating from VO2-Si hybrid coupling also observed in the insulating state. This offers a new strategy for the design of multifunctional terahertz metasurfaces.