Enhancing Near-Field\nRadiative Heat Transfer between\nDissimilar Dielectric Media by Coupling Surface Phonon Polaritons\nto Graphene’s Plasmons

Dielectric\nmedia are very promising for near-field radiative\nheat\ntransfer (NFRHT) applications as these materials can thermally emit\nsurface phonon polaritons (SPhPs) resulting in large and quasi-monochromatic\nheat fluxes. Near-field radiative heat flux between dissimilar dielectric\nmedia is much smaller than that between similar dielectric media and\nis also not quasi-monochromatic. This is due to the mismatch of the\nSPhP frequencies of the two heat-exchanging dielectric media. Here,\nwe experimentally demonstrate that NFRHT between dissimilar dielectric\nmedia increases substantially when a graphene sheet is deposited on\nthe medium with a smaller SPhP frequency. An enhancement of ∼2.7\nto 3.2 folds is measured for the heat flux between SiC and LiF separated\nby a vacuum gap of size ∼100–140 nm when LiF is covered\nby a graphene sheet. This enhancement is due to the coupling of SPhPs\nand surface plasmon polaritons (SPPs). The SPPs of graphene are coupled\nto the SPhPs of LiF resulting in coupled SPhP-SPPs with a dispersion\nbranch monotonically increasing with the wavevector. This monotonically\nincreasing branch of dispersion relation intersects the dispersion\nbranch of the SPhPs of SiC causing the coupling of the surface modes\nacross the vacuum gap, which resonantly increases the heat flux at\nthe SPhP frequency of SiC. This surface phonon-plasmon coupling also\nmakes NFRHT quasi-monochromatic, which is highly desired for applications\nsuch as near-field thermophotovoltaics and thermophotonics. This study\nexperimentally demonstrates that graphene is a very promising material\nfor tuning the magnitude and spectrum of NFRHT between dissimilar\ndielectric media.