Heat radiation and near-field radiative heat transfer can be strongly manipulated by adjusting geometrical shapes, optical properties, or the relative positions of the objects involved. Typically, these objects are considered as embedded in vacuum. By applying the methods of fluctuational electrodynamics, we derive general closed-form expressions for heat radiation and heat transfer in a system of N arbitrary objects embedded in a passive nonabsorbing background medium. Taking into account the principle of reciprocity, we explicitly prove the symmetry and positivity of transfer in any such system. Regarding applications, we find that the heat radiation of a sphere as well as the heat transfer between two parallel plates is strongly enhanced by the presence of a background medium. Regarding near-and far-field transfer through a gas like air, we show that a microscopic model (based on gas particles) and a macroscopic model (using a dielectric contrast) yield identical results. We also compare the radiative transfer through a medium like air and the energy transfer found from kinetic gas theory.

-We theoretically study heat transfer between two anisotropic nanoparticles in vacuum, and derive closed expressions in terms of the anisotropic dipole polarizabilities. We show that transfer between two small spheroids can be many times as large as the one for two spheres of same volumes. Such increase with anisotropy is also found for the heat emission of an isolated small spheroid. Furthermore, we observe a strong dependence of transfer on the relative orientation, yielding the interpretation as a heat transfer switch. The switch quality, given as the ratio of transfer in the "on" and "off" positions, is observed to be as large as 10 3 in the near field and even larger in the far field.The phenomenon of radiative heat transfer is of amplified interest due to recent experimental observations [1][2][3][4] of its strong increase for distances below the micron range. In this regime, transfer is enhanced by so called near-field effects attributed to evanescent waves [5]. On the theoretical side, two frameworks underlie thermal radiation and transfer, both of which are fundamental concepts. First, the theory of quantum thermal fluctuations that goes back all the way to the beginning of quantum mechanics, i.e., Planck's law of black body radiation [6]. Second, the scattering of light by objects that are small compared to the wavelength, which is by itself a modern field of both experimental and theoretical study [7,8].Radiative energy exchange between objects at different temperatures is on the macro scale well understood in terms of the laws by Planck and Stefan Boltzmann and inclusion of view factors and gray factors [9] to account for non-planar geometries and non-black bodies, respectively. However, such heat transfer is distinctly different, if the size of the objects or the distance between them is small or comparable to the thermal wavelength, which is roughly 8 microns at room temperature. On these scales, also nontrivial dependencies on the shape of the objects have been observed, as e.g. for sharp tips [10]. Many recent works computed the exact heat transfer between non planar objects including two spheres [11,12] or a sphere [10,13,14] or cone [10] in front of a planar surface, periodic structures [15], or even more abstract geometries [16,17]. Formalisms for treating fluctuation electrodynamics for arbitrary objects at different temperatures have been recently presented [13,16,18,19].Due to theoretical simplicity, a large influence on understanding was provided by the study of nano-particles, i.e., particles much smaller than the wavelength [20][21][22], including many body effects [23]. Such particles are accesibble experimentally. A recent work studies transfer between an anisotropic nano-particle (a spheroid) and a planar surface [24] (and the related Casimir interactions between small ellipsiods are analyzed in Ref. [25]).In this letter, we study radiative heat transfer between two anisotropic particles, as for example spheroids. We show that the transfer between two spheroids as well as the...

In this paper we discuss and review several aspects of the effect of boundary conditions and structured environments on dispersion and resonance interactions involving atoms or molecules, as well as on vacuum field fluctuations. We first consider the case of a perfect mirror, which is free to move around an equilibrium position and whose mechanical degrees of freedom are treated quantum mechanically. We investigate how the quantum fluctuations of the mirror's position affect vacuum field fluctuations for both a one-dimensional scalar and electromagnetic field, showing that the effect is particularly significant in the proximity of the moving mirror. This result can be also relevant for possible gravitational effects, since the field energy density couples to gravity. We stress that this interaction-induced modification of the vacuum field fluctuations can be probed through the Casimir-Polder interaction with a polarizable body, thus allowing to detect the effect of the mirror's quantum position fluctuations. We then consider the effect of an environment such as an isotropic photonic crystal or a metallic waveguide, on the resonance interaction between two entangled identical atoms, one excited and the other in the ground state. We discuss the strong dependence of the resonance interaction with the relative position of the atomic transition frequency with the gap of the photonic crystal in the former case, and with the cut-off frequency of waveguide in the latter.