  We compare super-Planckian thermal radiation between phonon-polaritonic media
and hyperbolic metamaterials. In particular, we determine the penetration depth
of thermal photons inside the absorbing medium for three different structures:
two semi-infinite phonon-polaritonic media supporting surface modes, two
multilayer hyperbolic metamaterials and two nanowire hyperbolic metamaterials.
We show that for hyperbolic modes the penetration depth can be orders of
magnitude larger than for surface modes suggesting that hyperbolic materials
are much more preferable for near-field thermophotovoltaic applications than
pure phonon-polaritonic materials.
