Reconfigurable terahertz-infrared absorber enabled by MEMS actuation and VO₂ phase transition

We propose a reconfigurable terahertz-infrared absorber based on boundary-condition-controlled mode switching in a MEMS-enabled VO₂-graphene Fabry-Pérot architecture. The VO₂ phase transition is used to selectively activate two distinct resonance families within the same device. In the insulating state, the absorber supports lateral Fabry-Pérot resonances and a hybrid guided-mode-like resonance associated with the graphene-dielectric optical stack. Their resonance frequencies are mainly determined by in-plane phase-matching conditions and remain nearly insensitive to cavity-depth variation, whereas the absorptance can be strongly modulated by changing the vertical field overlap with graphene. The representative L-FP mode exhibits an absorptance modulation of 99.2% and an amplitude-modulation dominance ratio of R_A = 209.5. In the metallic state, VO₂ acts as a reflective boundary and forms an effective vertical cavity with the Au reflector, activating QV-FP resonances whose frequencies are strongly tuned by the cavity depth. The representative QV-FP mode shows a frequency tuning range of 79.0% and a frequency-tuning dominance ratio of R_f = 2.633. Peak absorptance values reach 99.61% in the narrowband regime and 99.996% in the QV-FP regime. This boundary-dependent transition between lateral-resonance-dominated and vertical-cavity-dominated regimes provides a versatile strategy for multifunctional THz-IR absorbers with switchable amplitude- and frequency-tuning functionalities.