ABSTRACT 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‐MoO 3 nanocavities on phase‐change VO 2 at varying temperatures. At 25°C (VO 2 insulating phase), adjusting incident light frequency tunes the highly sensitive Fabry–Pérot phononic polaritonic resonator order from 10 to 2; at 90°C (VO 2 metallic phase), the order tunes from 7 to 2. With incident light fixed at 992 cm −1 , the α‐MoO 3 nanocavity/VO 2 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‐MoO 3 nanocavities. This method provides new means and experimental support for designing tunable Fabry–Pérot resonant devices.
Shi et al. (2026) studied this question.