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.Comment: 14 pages, 4 figure