  The inherent crystal anisotropy of hexagonal boron nitride (hBN) sustains
naturally hyperbolic phonon polaritons, i.e. polaritons that can propagate with
very large wavevectors within the material volume, thereby enabling optical
confinement to exceedingly small dimensions. Indeed, previous research has
shown that nanometer-scale truncated nanocone hBN cavities, with deep
subwavelength dimensions, support three-dimensionally confined optical modes in
the mid-infrared. Due to optical selection rules, only a few of many such modes
predicted theoretically have been observed experimentally via far-field
reflection and scattering-type scanning near-field optical microscopy. The
Photothermal induced resonance (PTIR) technique probes optical and vibrational
resonances overcoming weak far-field emission by leveraging an atomic force
microscope (AFM) probe to transduce local sample expansion due to light
absorption. Here we show that PTIR enables the direct observation of previously
unobserved, dark hyperbolic modes of hBN nanostructures. Leveraging these
optical modes could yield a new degree of control over the electromagnetic
near-field concentration, polarization and angular momentum in nanophotonic
applications.
