  Phonon-polaritons, mixed excitations of light coupled to lattice vibrations
(phonons), are emerging as a powerful platform for nanophotonic applications.
This is because of their ability to concentrate light into extreme
sub-wavelength scales and because of their longer phonon lifetimes than their
plasmonic counterparts. In this work, the infrared properties of
phonon-polaritonic nanoresonators made of monoisotopic B-10 hexagonal-boron
nitride (h-BN) are explored, a material with increased phonon-polariton
lifetimes compared to naturally abundant h-BN due to reduced photon scattering
from randomly distributed isotopes. An average relative improvement of 50% in
the nanoresonators Q factor is obtained with respect of nanoresonators made of
naturally abundant h-BN. Moreover, the monoisotopic h-BN nano-ribbon arrays are
used to sense nanometric-thick films of molecules, both through
surface-enhanced absorption spectroscopy and refractive index sensing. In
addition, strong coupling is achieved between a molecular vibration and the
phonon-polariton resonance in monoisotopic h-BN ribbons.
