  The physical origin of spectral shift of thermally generated surface
phonon-polariton (SPhP) resonance of a silicon carbide (SiC) bulk mediated by a
non-resonant film is elucidated. The local density of electromagnetic states
(LDOS) in a non-resonant intrinsic silicon (Si) film due to thermal emission by
SiC, derived using fluctuational electrodynamics, exhibits a local maximum near
SPhP resonant frequency in addition to a lower frequency resonance generated by
gap modes emerging in the vacuum gap separating the SiC and Si layers. Multiple
reflections within the vacuum gap also induce a LDOS drop around SPhP resonant
frequency. As a result, depending on the film thickness to vacuum gap ratio and
the location where the LDOS is calculated in the film, the low-frequency
resonance can dominate the LDOS, such that SPhP resonance appears to be
redshifted. A similar spectral behavior is observed on the monochromatic
radiative heat flux absorbed by the Si film. It is shown that apparent spectral
(red and blue) shift of SPhP resonance mediated by a non-resonant film is
bounded by the transverse and longitudinal optical phonon frequencies of SiC.
This work is of importance in applications involving dissimilar materials, such
as thermophotovoltaics and thermal rectification, where gap modes may
significantly disrupt flux resonance. Gap modes may also be at the origin of
the resonance redshift systematically observed in near-field thermal
spectroscopy.
