Reversible and Dynamic Tuning Mechanism for Fabry-Pérot Polaritonic Resonators

Polaritons are quasiparticles formed via light-matter interaction (e.g., with electrons or phonons). They confine light to subwavelength scales, enhance electromagnetic fields in Fabry-Pérot polaritonic resonators, and tune cavity resonance via adjusting external parameters (light frequency or dielectric environment). Thus, we propose tuning Fabry-Pérot polaritonic resonators by modifying the dielectric environment through substrate phase transitions. We perform mid-infrared nanoimaging of α-MoO₃ nanocavities on phase-change VO₂ at varying temperatures. At 25°C (VO₂ insulating phase), adjusting incident light frequency tunes the highly sensitive Fabry-Pérot phononic polaritonic resonator order from 10 to 2; at 90°C (VO₂ metallic phase), the order tunes from 7 to 2. With incident light fixed at 992 cm^-1, the α-MoO₃ nanocavity/VO₂ heterojunction undergoes a low-high-low temperature cycle, showing a maximum Fabry-Pérot resonance order change of 3, along with reversible tuning ability and delayed recovery. Simulations indicate higher-order Fabry-Pérot resonant modes can be achieved in thinner and wider α-MoO₃ nanocavities. This method provides new means and experimental support for designing tunable Fabry-Pérot resonant devices.