Probing individual phonon-polaritonic nanoparticle-on-mirror cavities by infrared nanospectroscopy

Nanoparticle-on-mirror (NPoM) cavities enable extreme light confinement and strong light-matter interactions, but their realization with phonon-polariton materials in the mid-infrared spectral range remains largely unexplored. Here, we use nano-FTIR spectroscopy to study the near-field response of individual phononic NPoM cavities formed by gold nanoparticles on a quartz substrate supporting phonon-polaritons. By placing a metal tip on top of the NPoM and recording the tip-scattered field, we observe two reproducible cavity resonances, identified as the fundamental and a second-order antenna modes by numerical simulations. The calculations show that, in absence of the tip, the NPoM cavity exhibits ultrasmall mode volumes ($V \sim 10^3$ nm$^3$) and high quality factors ($Q \sim 100$), resulting in extraordinary field intensity enhancements ($F \sim 10^4$) and Purcell factors ($P_F \sim 10^9$). They also indicate that the nano-FTIR tip enables efficient excitation and readout of these phononic NPoM modes without perturbing them, while enhancing the intrinsic local field intensity in the NPoM gap by two orders of magnitude. Our results establish phononic NPoM cavities as a promising platform for mid-infrared nanophotonics and pave the way for ultrasensitive vibrational spectroscopy based on nano-FTIR measurements of individual cavities.