Thermal near-field radiation of artificial materials

Heat radiation emitted by a medium with a given temperature depends on its material properties, and on its geometry as well. Artificial materials such as nanoporous media, photonic crystals, meta-materials or hyperbolic media feature very unusual optical properties, because of the combined effect of the properties of the respective material itself and of its geometrical structure. For example, by drilling nanoscale holes into a SiC or Au slab, or by ruling a nanoscale grating onto a SiC or Au surface, one can change the optical properties dramatically. This change, in its turn, can enhance or inhibit the heat flow caused by thermal radiation between such media. Therefore, artificial materials allow one to taylor the heat flux. We are investigating not only how the heat flux at the nanoscale can be controlled by such structures, but we also try to give an answer to a much more fundamental question: How does a black body for nanoscale heat radiation look like? And is it possible to artifically manufacture a black body for near-field heat radiation?

transmission coefficient for nanoporous materials

Thermal imaging at the nanoscale

We develop a theoretical description of the heat flux between a miniaturized sensing tip and a structured surface of different temperature at nanometer distances. A near-field effect known as “photon tunneling” then allows one to obtain images of the surface. Because such a “photon tunneling microscope” is sensitive to the electromagnetic local density of states, the information acquired in this manner differs from that accessible to a conventional electron tunneling microscope, which maps the electronic local density of states.

 

This work is performed in close collaboration with the experimental Energy and Semiconductor Research Group headed by Jürgen Parisi. Under the guidance of Achim Kittel, this group has developed a functioning Near-Field Scanning Thermal Microscope (NSThM), which has the unique capability of assessing the extreme near-field regime. Our joint Oldenburg Project aims at a comprehensive understanding of the working principles of this novel instrument. The figures below show a theoretically reconstructed (left) and an experimentally measured (right) thermal image of a nanostructured surface.

Comparision of theory and experiment

Cooling of nanoparticles

The heat radiation emitted by a nanoparticle depends strongly on its environment. As for the spontaneous emission of atoms, the heat radiated by a nanoparticle changes tremendously when placing it close to a cold surface. This effect is due to the near-field interaction between the nanoparticle and the surface. This near-field interaction is sensitive to the geometry of the particle (that is, whether its shape is rice corn-like or pancake-like) and to that of the surface (also including surface roughness). In addition, it turns out that many-particle effects (configurational resonances) affect the heat flux between nanoparticles. For instance, the radiative heat flow between two nanoparticles can be controlled (increased and decreased) by introducing a third nanoparticle, due to such configurational resonances.

Heat flux 'through' a third particle