A Green's function formalism of energy and momentum transfer in fluctuational electrodynamics

Radiative energy and momentum transfer due to fluctuations of electromagnetic fields arising due to temperature difference between objects is described in terms of the cross-spectral densities of the electromagnetic fields. We derive relations between thermal non-equilibrium contributions to energy and momentum transfer and surface integrals of tangential components of the dyadic Green's functions of the vector Helmholtz equation. The expressions derived here are applicable to objects of arbitrary shapes, dielectric functions, as well as magnetic permeabilities. For the case of radiative transfer, we derive expressions for the generalized transmissivity and generalized conductance that are shown to obey reciprocity and agree with theory of black body radiative transfer in the appropriate limit.Comment: 12 pages, 2 figure

Lifshitz theory of van der Waals pressure in dissipative media

We derive a first--principles method of determining the van der Waals or Casimir pressure in a dissipative and dispersive planar multilayered system by calculating the Maxwell stress tensor in a fictitious layer of vacuum, that is eventually made to vanish, introduced in the structure. This is illustrated by calculating the van der Waals pressure in a thin film with dissipative properties embedded between two semi--infinite media.Comment: 4 pages, 2 figure

Surface Modes for Near Field Thermophotovoltaics

Thermal radiative energy transfer between closely spaced surfaces has been analyzed in the past and shown not to obey the laws of classical radiation heat transfer owing to evanescent waves and, more recently, electromagnetic surface modes. We have analyzed the energy transfer between layered media, one of the layers being the thermal source, using a Green’s functions method and the fluctuation-dissipation theorem. Based on the analysis, we propose a structure that can utilize the surface modes to increase the power density and efficiency of low temperature thermophotovoltaic generators. © 2003 American Institute of Physics

Convergence of Vector Spherical Wave Expansion Method Applied to Near-Field Radiative Transfer

Near-field radiative transfer between two objects 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 the dyadic Green’s function of the vector Helmholtz equation. When the two objects are spheres, the dyadic Green’s 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

Convergence of Vector Spherical Wave Expansion Method Applied to Near-Field Radiative Transfer

Near-field radiative transfer between two objects 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 the dyadic Green’s function of the vector Helmholtz equation. When the two objects are spheres, the dyadic Green’s 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

Thermal Near-field Radiative Transfer Between Two Spheres

Radiative energy transfer between closely spaced bodies is known to be significantly larger than that predicted by classical radiative transfer because of tunneling due to evanescent waves. Theoretical analysis of near-field radiative transfer is mainly restricted to radiative transfer between two half-spaces or spheres treated in the dipole approximation (very small sphere) or proximity force approximation (radius of sphere much greater than the gap). Sphere-sphere or sphere-plane configurations beyond the dipole approximation or proximity force approximation have not been attempted. In this work, the radiative energy transfer between two adjacent non-overlapping spheres of arbitrary diameters and gaps is analyzed numerically. For spheres of small diameter (compared to the wavelength), the results coincide with the dipole approximation. We see that the proximity force approximation is not valid for spheres with diameters much larger than the gap, even though this approximation is well established for calculating forces. From the numerical results, a regime map is constructed based on two nondimensional length scales for the validity of different approximations

Proximity Effects in Radiative Heat Transfer

Though the dependence of near-field radiative transfer on the gap between two planar objects is well understood, 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