  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.
