Nonlinear Radiation at the Nanoscale

Abstract

All matter emits electromagnetic radiation arising from the thermal motion of charged particles or ultimately, internal energy level transitions. A century of work on such radiative processes has culminated in greater understanding and hence control of radiation, paving the way for many technological applications. And yet, there is plenty of room at the nanoscale to further refine our understanding and discover new ways of taming radiation. In this dissertation, we go beyond traditional systems, which have primarily focused on linear passive media, to investigate the impact of nonlinearities (the major focus of this work) and optically active gain media on radiation.

We explore nanophotonic resonant systems since recent advances have achieved resonantly enhanced nonlinear interactions of light via ultra-small confinements and ultra-high lifetimes (quality factors) and made nonlinear effects accessible to low-power phenomena like thermal radiation. We find that resonant cavities containing Kerr $\chi^{(3)}$ nonlinear medium exhibit intriguing features such as

spectral alterations, greater than linear black-body emission under nonequilibrium conditions, and thermally activated transitions. We

further analyze a nonlinear upconversion scheme that resonantly enhances $\chi^{(3)}$ four-wave mixing between mid-infrared thermal and pump photons and yields a large density of near-infrared thermal photons. Such a process paves the way to significantly enhance otherwise exponentially suppressed, room temperature emission at near-infrared frequencies.

Radiative heat transfer between two bodies separated by sub-micron gaps (near-field) is enhanced due to interference of evanescent and surface waves. While such enhancements are typically studied in symmetric configurations of vacuum separated slabs held under large temperature differentials, we employ the four-wave mixing scheme to achieve comparable or even larger flux rates between slabs of dissimilar (mid-infrared and near-infrared) polaritonic wavelengths under arbitrary (even zero) temperature differentials. As an

alternative mechanism of control in the presence of gain media, we also demonstrate that a proper combination of loss-cancellation due to gain and geometry-dependent proximity interactions results in orders of magnitude larger near-field enhancements (diverging close to the lasing threshold) compared to typically considered passive scenarios.

Finally, based on the above and other related ideas, we propose potential thermal applications, which include thermal refrigeration,

tunable near-field heat exchange, and thermal bistability in three-body configurations.