  A promising method to leverage near-field power densities without the use of
nanoscale vacuum gaps is through hyperbolic metamaterial (HMM) waveguides. When
placed between a hot and cold reservoir, an ideal HMM can transmit surface
waves across several microns, enabling an extension of near-field enhancements.
However, when accounting for transmission loss due to realistic levels of
absorption within the waveguide, previous studies have shown that the
enhancements are significantly curtailed at wide separations. In our study, we
investigate the role of internal sources within realistic non-isothermal HMMs.
We demonstrate that, in some cases, the emission from the HMM accounts for over
90% of the total heat transfer to the receiver, and that these additional
sources can largely compensate for optical losses associated with decreased
transmission from the emitter to the receiver. Lastly, we investigate the
spectral transport in a realistic 3-body system which has mismatching optical
properties between the boundaries (emitter and receiver) and the waveguide
(HMM). Our model shows that the near-field thermal transport remains spectrally
selective to the boundaries, even as major radiative contributions come from
the waveguide. This work may enable the design of non-isothermal
emitter-waveguide-receiver systems that transmit near-field power levels over
wider separations.
