Karthik Sasihithlu

Proximity Effects in Radiative Heat Transfer

Thoughthedependenceofnear-fieldradiativetransferonthegapbetweentwoplanarobjectsiswellunderstood, that between curved objects is still unclear. We show unequivocally that the surface polariton mediated radiative transfer between two spheres of equal radii R and minimum gap d scales as R/d as the nondimensional gap d/R → 0. We discuss the proximity approximation form that is being used at present to compare with experimental observations and suggest a modified form in order to satisfy the continuity requirement between far-field and near-field radiative transfer between the spheres.

Convergence of vector spherical wave expansion method applied to near-field radiative transfer.

Near-field radiative transfer between two objects can be computed using Rytovs theory of fluctuational electrodynamics in which the strength of electromagnetic sources is related to temperature through the fluctuation-dissipation theorem, and the resultant energy transfer is described using the dyadic Greens function of the vector Helmholtz equation. When the two objects are spheres, the dyadic Greens function can be expanded in a series of vector spherical waves. Based on comparison with the convergence criterion for the case of radiative transfer between two parallel surfaces, we derive a relation for the number of vector spherical waves required for convergence in the case of radiative transfer between two spheres. We show that when electromagnetic surface waves are active at a frequency the number of vector spherical waves required for convergence is proportional to Rmax/d when d/Rmax → 0, where Rmax is the radius of the larger sphere, and d is the smallest gap between the two spheres. This criterion for convergence applies equally well to other near-field electromagnetic scattering problems.

Near-field radiative transfer between two unequal sized spheres with large size disparities

We compute near-field radiative transfer between two spheres of unequal radii R1 and R2 such that R2 ≲ 40R1. For R2 = 40R1, the smallest gap to which we have been able to compute radiative transfer is d = 0.016R1. To accomplish these computations, we have had to modify existing methods for computing near-field radiative transfer between two spheres in the following ways: (1) exact calculations of coefficients of vector translation theorem are replaced by approximations valid for the limit d ≪ R1, and (2) recursion relations for a normalized form of translation coefficients are derived which enable us to replace computations of spherical Bessel and Hankel functions by computations of ratios of spherical Bessel or spherical Hankel functions. The results are then compared with the predictions of the modified proximity approximation.

Near Field Radiative Heat Transfer Measurement

Using an improved experimental setup, we measured near field radiative heat transfer that may benefit TPV up to a value as small as 0.5 nW/K. The experimental data is compared with modified proximity approximation prediction

Van der Waals Force Assisted Heat Transfer

Abstract Phonons (collective atomic vibrations in solids) are more effective in transporting heat than photons. This is the reason why the conduction mode of heat transport in nonmetals (mediated by phonons) is dominant compared to the radiation mode of heat transport (mediated by photons). However, since phonons are unable to traverse a vacuum gap (unlike photons), it is commonly believed that two bodies separated by a gap cannot exchange heat via phonons. Recently, a mechanism was proposed [J. B. Pendry, K. Sasihithlu, and R. V. Craster, Phys. Rev. B 94, 075414 (2016)] by which phonons can transport heat across a vacuum gap – through the Van der Waals interaction between two bodies with gap less than the wavelength of light. Such heat transfer mechanisms are highly relevant for heating (and cooling) of nanostructures; the heating of the flying heads in magnetic storage disks is a case in point. Here, the theoretical derivation for modelling phonon transmission is revisited and extended to the case of two bodies made of different materials separated by a vacuum gap. Magnitudes of phonon transmission, and hence the heat transfer, for commonly used materials in the micro- and nano-electromechanical industry are calculated and compared with the calculation of conduction heat transfer through air for small gaps as well as the heat transfer calculation due to photon exchange.

A surface-scattering model satisfying energy conservation and reciprocity

Abstract Roughness scattering models based on Kirchhoff׳s approximation or perturbation theory give a good account of the angular distribution of the scattered intensity but do not satisfy energy conservation and reciprocity rigorously. For applications such as solar cells with rough interfaces producing a quasi isotropic intensity in the multiple scattering regime, an accurate model of the angular pattern is not required. Instead, energy conservation and reciprocity must be satisfied with great accuracy. Here we present a surface scattering model based on analysis of scattering from a layer of particles on top of a substrate in the dipole approximation which satisfies both energy conservation and reciprocity and is thus accurate in all frequency ranges. The model takes into account the absorption in the substrate induced by the particles but does not take into account the near-field interactions between the particles. In arriving at this model, we use the effective-medium approach to show how we can proceed from modeling the electromagnetic scattering from a single particle to modeling the scattering from a layer of particles positioned above a substrate, and finally relate this to the bidirectional scattering distribution function of the substrate.

Non-Surface Polaritonic Peaks in Near-Field Radiative Transfer

Near-field effects in radiative transfer refer to the collective influence of interference, diffraction, and tunneling of electro-magnetic waves on energy transfer between two or more objects. Most studies of near-field radiative transfer have so far focused on the enhancement due to tunneling of surface polaritons. In this work, we show the existence of sharp peaks in the radiative transfer spectrum between two spheres of polar materials that are not due to surface polaritons. The peaks, which are present on either side of the restrahlen band, are because of Mie resonances.Copyright

Effect of Curvature on Near-Field Radiative Transfer: The Modified Proximity Approximation

Near-field radiative transfer between two spheres can be computed using Rytov’s theory of fluctuational electrodynamics in which the strength of electromagnetic sources is related to temperature through the fluctuation-dissipation theorem, and the resultant energy transfer is described using an expansion of the dyadic Green’s function of the vector Helmholtz equation in a series of vector spherical waves. We show that when electromagnetic surface waves are active at a frequency the number of vector spherical waves required for convergence is proportional to Rmax/d when d/Rmax → 0, where Rmax is the radius of the larger sphere, and d is the smallest gap between the two spheres. Using this criterion, we show that the surface polariton mediated near–field thermal radiative conductance between two spheres of equal radii R scales as R/d as d/R → 0. We also propose a modified form of the proximity approximation to predict near–field radiative transfer between curved objects from simulations of radiative transfer between parallel surfaces.Copyright

Measurement of Near Field Radiation Between a Microsphere and an Infinite Plane With Improved Optical Beam

Classical theory of thermal radiation can be used to describe radiative transfer between two objects when the characteristic length scales, such as linear dimensions of the objects and inter-object separation, are much longer than the characteristic thermal wave length given by Wien’s displacement law. In situations where the separation is comparable or smaller than the characteristic wave length, near field effects need to be taken into account. Near field radiative transfer can be enhanced by several orders of magnitude if the materials of the objects support surface phonon polaritons. Although near field radiation between two parallel surfaces have been well studied theoretically, the difficulties in probing radiative transfer between parallel surfaces with nanoscale gaps have prevented meaningful experimental validation. In recent years several experiments have measured near field radiation beyond blackbody limitation between a microsphere and a substrate. In those experiments employing optical beam deflection technique, the sphere has to be placed near the edge of the substrate in order to prevent unintentional chopping of the laser beam by the edge of the substrate. The actual measurements are performed between a sphere and a semi-infinite plane instead of an infinite plane. We report here an improved optical beam deflection system to measure the near field radiative heat transfer between a sphere and a substrate. With the new setup, the sphere can be placed sufficiently far away from the substrate edge, rendering it a better approximation of a sphere and an infinite plane. The experimental results and the numerical prediction using modified proximity approximation will be discussed.© 2012 ASME