  Hyperbolic phonon polaritons (HPhPs) are generated when infrared photons
couple to polar optic phonons in anisotropic media, confining long-wavelength
light to nanoscale volumes. However, to realize the full potential of HPhPs for
infrared optics, it is crucial to understand propagation and loss mechanisms on
substrates suitable for applications from waveguiding to infrared sensing. In
this paper, we employ scattering-type scanning near-field optical microscopy
(s-SNOM) and nano-Fourier transform infrared (FTIR) spectroscopy, in concert
with analytical and numerical calculations, to elucidate HPhP characteristics
as a function of the complex substrate dielectric function. We consider
propagation on suspended, dielectric and metallic substrates to demonstrate
that the thickness-normalized wavevector can be reduced by a factor of 25
simply by changing the substrate from dielectric to metallic behavior.
Moreover, by incorporating the imaginary contribution to the dielectric
function in lossy materials, the wavevector can be dynamically controlled by
small local variations in loss or carrier density. Such effects may therefore
be used to spatially separate hyperbolic modes of different orders, and
indicates that for index-based sensing schemes that HPhPs can be more sensitive
than surface polaritons in the thin film limit. Our results advance our
understanding of fundamental polariton excitations and their potential for
on-chip photonics and planar metasurface optics.
