  Localized and propagating polaritons allow for highly sensitive analysis of
(bio)chemical substances and processes. Nanoimaging of the polaritons
evanescent fields allows for critically important experimental mode
identification and for studying field confinement. Here we describe two setups
for polariton nanoimaging and spectroscopy in liquid, which is an indispensable
environment for (bio)chemical samples. We first demonstrate antenna mapping
with a transflection infrared scattering-type scanning near-field optical
microscope (s-SNOM), where the tip acts as a near-field scattering probe. We
then demonstrate a total internal reflection (TIR) based setup, where the tip
is both launching and probing ultra-confined polaritons in van der Waals
materials, here phonon polaritons in hexagonal boron nitride (h-BN) flakes.
This work lays the foundation for s-SNOM based polariton interferometry in
liquid, which has wide application potential for in-situ studies of chemical
reactions at the bare or functionalized surface of polaritonic materials,
including (bio)chemical recognition analogous to the classical surface plasmon
resonance spectroscopy.
