Direct calculation of energy streamlines in near-field thermal radiation

Accurate prediction of the energy propagation direction and the associated lateral shift is very important for the design of efficient energy conversion devices and sensors based on nanoscale thermal radiation. This paper concerns the direction of energy flow during near-field radiative transfer between two parallel plates separated by a vacuum gap. An improved formulation, fully consistent with fluctuational electrodynamics, is developed to correctly trace the energy streamlines inside the emitter, receiver, and the vacuum gap. The influence of surface waves on the direction of energy propagation as well as on the lateral shift of energy streamlines is elucidated. An important finding with the improved formulation is that the lateral shift in the emitter may exceed that in the vacuum gap. The method can be extended for tracing the energy streamlines in multilayered structures.