  Phonon polaritons (PhPs), the collective phonon oscillations with hybridized
electromagnetic fields, concentrate optical fields in the mid-infrared
frequency range that matches the vibrational modes of molecules. The
utilization of PhPs holds the promise for chemical sensing tools and
polariton-enhanced nanospectroscopy. However, investigations and innovations on
PhPs in the aqueous phase remains stagnant, because of the lack of in situ
mid-infrared nano-imaging methods in water. Strong infrared absorption from
water prohibits optical delivery and detection in the mid-infrared for
scattering-type near-field microscopy. Here, we present our solution: the
detection of photothermal responses caused by the excitation of PhPs by liquid
phase peak force infrared (LiPFIR) microscopy. Characteristic interference
fringes of PhPs in 10B isotope-enriched h-BN were measured in the aqueous phase
and their dispersion relationship extracted. LiPFIR enables the measurement of
mid-infrared PhPs in the fluid phase, opening possibilities, and facilitating
the development of mid-IR phonon polaritonics in water.
