Tuning Vibrational Coupling Strength Between Hyperbolic Media and Organic Molecules

In the field of molecular polaritonics, stronger coupling between vibrational modes and cavity systems is highly sought after. The coupling strength in these systems scales inversely with cavity mode volumes (V_m), yet traditional Fabry-Perot cavities are diffraction-limited. To overcome this limitation, we employ hyperbolic hexagonal boron nitride (hBN) to enable subdiffractional Fabry-Perot-like modes. In this work, we use scattering-type scanning near-field optical microscopy (s-SNOM) to directly image polariton propagation and quantify vibrational strong coupling to weak C-H bending modes in poly(methyl methacrylate) (PMMA). The isotopic composition of the hBN layer dictates the frequency range of the material's Reststrahlen band, with the strongest coupling arising when the TO phonon is closest in energy to the molecular vibrational mode. We further characterize how coupling strengths scale with the thickness of the PMMA and hBN layers, as the PMMA layer thickness dictates the number of molecules available to fill an evanescent mode volume and the thickness of the hBN dictates the size of the evanescent mode volume. Ultimately, this study demonstrates that reduced mode volumes and a more complete understanding of the coupling mechanisms to tune and control the interactions are crucial to advancement in the field.