Theoretical description of a near-field scanning thermal microscope

In the last 10 years a near-field scanning thermal microscope (NSThM) has been developed in the group of Achim Kittel [1–3] at Oldenburg University. The NSThM allows for measuring near-field radiative heat fluxes in the ultrasmall distance regime of a few nanometer between a cold sample and the hot tip of the NSThM under ultrahigh vacuum conditions. In our presentation we will discuss the theoretical modelling of the radiative heat flux measured by the NSThM. We will show how we have applied Rytov's fluctuational electrodynamics [4] to model the signal of the NSThM for planar, structured or rough surfaces [5–8]. Our numerical results show a good qualitative agreement with the measurements of surface profiles [6]. However, a comparison of approximative and exact numerical theoretical results using proximity approximation [9] and a surface current approach [10, 11] with new measurements shows that there is a huge quantitative difference between the experimental data and the theoretical prediction [12]. This discrepancy indicates that Rytov's theory as it is commonly used is lacking an important heat flux channel or mechanism.