Enhancing Near-Field Radiative Heat Transfer between Dissimilar Dielectric Media by Coupling Surface Phonon Polaritons to Graphene’s Plasmons

Abstract

Dielectric media are very promising for near-field radiative heat transfer (NFRHT) applications as these materials can thermally emit surface phonon polaritons (SPhPs) resulting in large and quasi-monochromatic heat fluxes. Near-field radiative heat flux between dissimilar dielectric media is much smaller than that between similar dielectric media and is also not quasimonochromatic. This is due to the mismatch of the SPhP frequencies of the two heat-exchanging dielectric media. Here, we experimentally demonstrate that NFRHT between dissimilar dielectric media increases substantially when a graphene sheet is deposited on the medium with a smaller SPhP frequency. An enhancement of ∼2.7 to 3.2 folds is measured for the heat flux between SiC and LiF separated by a vacuum gap of size ∼100−140 nm when LiF is covered by a graphene sheet. This enhancement is due to the coupling of SPhPs and surface plasmon polaritons (SPPs). The SPPs of graphene are coupled to the SPhPs of LiF resulting in coupled SPhP-SPPs with a dispersion branch monotonically increasing with the wavevector. This monotonically increasing branch of dispersion relation intersects the dispersion branch of the SPhPs of SiC causing the coupling of the surface modes across the vacuum gap, which resonantly increases the heat flux at the SPhP frequency of SiC. This surface phonon-plasmon coupling also makes NFRHT quasi-monochromatic, which is highly desired for applications such as near-field thermophotovoltaics and thermophotonics. This study experimentally demonstrates that graphene is a very promising material for tuning the magnitude and spectrum of NFRHT between dissimilar dielectric media.