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Surface-plasmon-enhanced near-field radiative heat transfer between planar surfaces with a thin-film plasmonic coupler

In last decade, there have been enormous efforts to experimentally show the near-field enhancement of radiative heat transfer between planar structures. Several recent experiments also have striven to achieve further enhanced heat transfer with the excitation of coupled surface polaritons by introducing nanostructures on both emitter and the receiver; however, these symmetric structures are hardly employed in real-world applications. Here, we demonstrate substantially increased near-field radiative heat transfer between asymmetric structures (i.e., doped Si and SiO$_2$) by using a thin Ti film as a plasmonic coupler. The measured near-field enhancement at vacuum gap of 380 nm is found to be 3.5 times greater than that for the case without the coupler. The enhancement mechanism is thoroughly elucidated for both polarizations and a dimensionless parameter, which can quantify the coupling strength of the surface polaritons at vacuum, is suggested. As a thin film can be readily used in many engineering applications, this study will facilitate the development of the high-performance engineering devices exploiting the near-field thermal radiation.