Heat transfer between anisotropic nanoparticles: Enhancement and switching

Abstract

-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...

Limitations of kinetic theory to describe near-field heat exchanges in many-body systems

“…Beside this approximate theory a rigorous strategy has been followed in 2011 to describe the near-field heat transfers in N -body systems consisting of small size body [11][12][13][14][15][16][17] and later to describe more general situations with systems of arbitrary size and of various geometries 18,19 by generalizing Rytov's theory to manybody systems allowing also to take into account the spatial distribution of temperature profiles 20 . Following these theoretical developments numerous many-body effects have been highlighted in these systems.…”

Photon Thermal Hall Effect

“…Therefore a heat flux propagates transversally to the primary applied temperature gradient. Using the Landauer formalism for N-body systems [8][9][10][11][12][13] the heat flux exchanged between the i th and the j th particle in the network reads…”

Radiative heat transfer between nanoparticles: Shape dependence and three-body effect

“…This dependence appeared as Imχ 1 Imχ 2 in both two-and three-body systems, which can be optimized by tuning the overlap of polarizability tensor components even by shape or material. As shown by Incardone et.al, the heat transfer between two spheroids can be switched on/of by changing their relative orientation 49 . The presence of additional particle leads to significant modification in the transmission probability which depends in a complicated manner on shapes in the limit of small separations.…”