Transition from Near-Field to Extreme Near-Field Radiative Heat Transfer

The radiative heat transfer in the extreme near-field regime, i.e., for distances below 10 nm, remains poorly understood. There are competing experimental results in this regime, with some in good agreement with theoretical predictions, while others report drastically elevated heat fluxes, orders of magnitude larger than what theory suggests. Whether the theory of fluctuational electrodynamics can predict the radiative heat transfer in this extreme near-field regime or not remains a matter of active debate. In this Letter, a radiative heat transfer measurement is presented between a gold-coated sphere and a gold film sample in the transition regime between the near-field and the extreme near-field regime just before contact. The radiative heat flux measurement is made by using a near-field scanning thermal microscope equipped with a temperature sensor at a sharp tip as a heat flux sensor. We find an excellent agreement with the theoretical predictions of fluctuational electrodynamics in the near-field regime. In the extreme near-field regime, however, a highly increased radiative heat flux is observed with values about 100 times larger than the theoretical predictions, indicating that fluctuational electrodynamics fails to capture the radiative heat flux in this regime.