Boron nitride nanoresonators for phonon-enhanced molecular vibrational spectroscopy at the strong coupling limit

Abstract

Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and con- ventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanor- esonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high- quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hex- agonal boron nitride (h-BN). In this work we use, for the fi rst time, phonon-polariton-resonant h-BN ribbons for SEIRA spectro- scopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.