Highly Sensitive and Tunable Polaritonic Fabry–Pérot Nanoresonators of α-MoO 3

Hyperbolic phonon polaritons are hybrid light–matter excitations that exhibit strong light confinement and low propagation loss, making them promising for field-enhanced infrared spectroscopy and advanced on-chip optoelectronics. However, fundamental challenges persist in understanding and controlling polaritonic resonator characteristics, including mode formation mechanisms, quality factor optimization, and spectral tunability, which critically determine their practical applications. Here, we combine scattering-type scanning near-field optical microscopy (s-SNOM) experiments and a theoretical method to comprehensively study the polaritonic Fabry–Pérot resonators in α-MoO 3 nanocavities. The results reveal that the resonators exhibit tunable Fabry–Pérot modes with exceptional spectral sensitivity (1 cm –1 ), enabling tailored mid-infrared molecular spectroscopy. Moreover, our theoretical method can accurately predict quality factors based on the propagation loss of polaritons. These findings establish polaritonic nanocavities as versatile platforms for polariton-enhanced spectroscopy, bridging fundamental insights with practical nanophotonic applications.