Near-unity light-matter interaction in mid-infrared van der Waals nanocavities

Accessing mid-infrared radiation is of great importance for a range of applications, including thermal imaging, sensing, and radiative cooling. Here, we study light interaction with hexagonal boron nitride nanocavities and reveal strong and tunable resonances across its hyperbolic transition. In addition to conventional phonon-polariton excitations, we demonstrate that the high refractive index of hexagonal boron nitride outside the Reststrahlen band allows enhanced light-matter interactions in deep subwavelength (<{\lambda}/15) nanostructures across a broad 7-8 {\mu}m range. Near-unity absorption and high quality (Q>80) resonance interaction in the vicinity of the transverse optical phonon is observed. Our study provides new avenues to design highly efficient and ultracompact structures for controlling mid-infrared radiation and accessing strong light-matter interaction.