| Description |
Modeling techniques are provided for accurate and efficient solution of near-field radiative heat transfer in complex, three-dimensional and multiscale geometries. These techniques are applied to investigate the physics of near-field thermal radiation in several configurations. A closed-form expression based on fluctuational electrodynamics is derived and applied for modeling size effect on the emissivity of metallic and dielectric thin films. The emissivity of dielectric films increases with increasing film thickness, while metallic films show the inverse behavior. The critical thickness, above which no size effect is observed, is about a hundred nanometers for metals and a few centimeters for dielectrics. A novel computational method, called the thermal discrete dipole approximation (T-DDA), for modeling near-field radiative heat transfer in arbitrary geometries is proposed and verified. The T-DDA is based on discretizing objects into cubical subvolumes behaving as electric point dipoles. The objects are submerged in an infinite lossless medium and can interact with an infinite surface. An extensive convergence analysis of the method is performed using the exact results for two spheres. The convergence of the T-DDA mostly depends on the dielectric function of the objects and the object size to gap ratio. An error less than 5% was achievable in the T-DDA using the available computational resources. The T-DDA is applied to model near-field thermal radiation between a silica probe and a silica surface separated by a gap of size d. When d --> 0, the probe-surface heat rate is dominated by the contribution of surface phonon-polaritons and approaches a d^-2 power law. In this limit, the total heat rate and the resonance location can be predicted using the proximity approximation. When the probe tip size is comparable to the gap thickness, localized surface phonons also contribute to heat transfer and induce a resonance splitting in the thermal spectrum. In this regime, the spheroidal dipole approximation predicts the resonant frequencies accurately, and it provides a rough estimate of the heat rate. Finally, the T-DDA analysis of probe-sample interactions demonstrates that the resonance redshift observed in near-field thermal spectroscopy is caused by the reflection interactions between the probe and the sample. |
