Vibrational Strong Coupling in Nanoscale Hyperbolic Phonon Polariton Cavities

Vibrational strong coupling (VSC) between optical cavities and molecular vibrational modes has been demonstrated to change vibrational lifetimes and modify chemical reaction rates. Current VSC studies rely on traditional microscopic Fabry-Perot (FP) cavities, which limits their application to molecules with high oscillator strength, such as metal carbonyls. To expand VSC studies to a broader range of chemical systems, we explore the use of hyperbolic phonon polaritons (HPhPs) to dramatically reduce the cavity mode volume (CMV) and thus enhance light-matter interactions through deeply sub-diffractional confinement of light, as well as higher quality factors. To determine how VSC scales with CMV and oscillator strengths, we begin with microscale FP cavities of varying length and spin coat thin films of N-methylacetamide (NMA), the smallest amide molecule with a <i>trans</i>-peptide bond embedded into a polymer matrix onto the mirror surface. We then extend this framework to nanoscale arrays of HPhP cavities with a resonance overlapping the Amide I region in NMA to achieve VSC mode volume and strong coupling. By coupling HPhP cavities with two-dimensional infrared spectroscopy, we also explore the possible coupling rates for nanoscale HPhP cavities and determine the limits to which we can push CMV as VSC increases. Altogether, this approach lays the groundwork for future studies, which will focus on understanding and control of chemical reactions and molecular dynamics at low, biologically relevant concentrations.