Svend-Age Biehs

Near-field heat transfer in a scanning thermal microscope

We present measurements of the near-field heat transfer between the tip of a thermal profiler and planar material surfaces under ultrahigh vacuum conditions. For tip-sample distances below 10(-8) m, our results differ markedly from the prediction of fluctuating electrodynamics. We argue that these differences are due to the existence of a material-dependent small length scale below which the macroscopic description of the dielectric properties fails, and discuss a heuristic model which yields fair agreement with the available data. These results are of importance for the quantitative interpretation of signals obtained by scanning thermal microscopes capable of detecting local temperature variations on surfaces.

Hyperbolic Metamaterials as an Analog of a Blackbody in the Near Field

A black body is usually defined by its property of having a maximum absorptivity and therefore also a maximum emissivity by virtue of Kirchhoff’s law [1]. The energy transmission between two black bodies having different temperatures obey the well-known Stefan-Boltzmann law. This law sets an upper limit for the power which can be transmitted by real materials, but it is itself a limit for the far-field only, since it takes only propagating modes into account. In terms of the energy transmission between two bodies the black body case corresponds to maximum transmission for all allowed frequencies ω and all wave vectors smaller than ω/c, where c is the vacuum light velocity. This means that all the propagating modes are perfectly transmitted across the separation gap.

Near-field thermal transistor.

Using a block of three separated solid elements, a thermal source and drain together with a gate made of an insulator-metal transition material exchanging near-field thermal radiation, we introduce a nanoscale analog of a field-effect transistor that is able to control the flow of heat exchanged by evanescent thermal photons between two bodies. By changing the gate temperature around its critical value, the heat flux exchanged between the hot body (source) and the cold body (drain) can be reversibly switched, amplified, and modulated by a tiny action on the gate. Such a device could find important applications in the domain of nanoscale thermal management and it opens up new perspectives concerning the development of contactless thermal circuits intended for information processing using the photon current rather than the electric current.

Phase-change radiative thermal diode

A thermal diode transports heat mainly in one preferential direction rather than in the opposite direction. This behavior is generally due to the non-linear dependence of certain physical properties with respect to the temperature. Here we introduce a radiative thermal diode which rectifies heat transport thanks to the phase transitions of materials. Rectification coefficients greater than 70% and up to 90% are shown, even for small temperature differences. This result could have important applications in the development of future contactless thermal circuits or in the conception of radiative coatings for thermal management.

Mesoscopic description of radiative heat transfer at the nanoscale.

We present a formulation of the nanoscale radiative heat transfer using concepts of mesoscopic physics. We introduce the analog of the Sharvin conductance using the quantum of thermal conductance. The formalism provides a convenient framework to analyze the physics of radiative heat transfer at the nanoscale. Finally, we propose a radiative heat transfer experiment in the regime of quantized conductance.

Nanoscale heat flux between nanoporous materials.

By combining stochastic electrodynamics and the Maxwell-Garnett description for effective media we study the radiative heat transfer between two nanoporous materials. We show that the heat flux can be significantly enhanced by air inclusions, which we explain by: (a) the presence of additional surface waves that give rise to supplementary channels for heat transfer throughout the gap, (b) an increase in the contribution given by the ordinary surface waves at resonance, (c) and the appearance of frustrated modes over a broad spectral range. We generalize the known expression for the nanoscale heat flux for anisotropic metamaterials.

Super-Planckian near-field thermal emission with phonon-polaritonic hyperbolic metamaterials

We study super-Planckian near-field heat exchanges for multilayer hyperbolic metamaterials using exact scattering-matrix (S-matrix) calculations. We investigate heat exchanges between two multilayer hyperbolic metamaterial structures. We show that the super-Planckian emission of such metamaterials can either come from the presence of surface phonon-polariton modes or from a continuum of hyperbolic modes depending on the choice of composite materials as well as the structural configuration.

Modulation of near-field heat transfer between two gratings

We present a theoretical study of near-field heat transfer between two uniaxial anisotropic planar structures. We investigate how the distance and relative orientation (with respect to their optical axes) between the objects affect the heat flux. In particular, we show that by changing the angle between the optical axes it is possible in certain cases to modulate the net heat flux up to 90% at room temperature, and discuss possible applications of such a strong effect.

Many-body radiative heat transfer theory.

In this Letter, an N-body theory for the radiative heat exchange in thermally nonequilibrated discrete systems of finite size objects is presented. We report strong exaltation effects of heat flux which can be explained only by taking into account the presence of many-body interactions. Our theory extends the standard Polder and van Hove stochastic formalism used to evaluate heat exchanges between two objects isolated from their environment to a collection of objects in mutual interaction. It gives a natural theoretical framework to investigate the photon heat transport properties of complex systems at the mesoscopic scale.

Thermal heat radiation, near-field energy density and near-field radiative heat transfer of coated materials

Abstract.We investigate the thermal radiation and thermal near-field energy density of a metal-coated semi-infinite body for different substrates. We show that the surface polariton coupling within the metal coating leads to an enhancement of the TM-mode part of the thermal near-field energy density when a polar substrate is used. In this case the result obtained for a free standing metal film is retrieved. In contrast, in the case of a metal substrate there is no enhancement in the TM-mode part, as can also be explained within the framework of surface plasmon coupling within the coating. Finally, we discuss the influence of the enhanced thermal energy density on the near-field radiative heat transfer between a simple semi-infinite and a coated semi-infinite body for different material combinations.