Fluctuational electrodynamics for nonlinear media (Vol. 48 No. 2)

Nonlinear coupling of incident waves (blue) and fluctuation-induced waves (black)

Nonlinear optics gives rise to a lot of interesting phenomena like frequency mixing, the optical Kerr effect, the Raman effect, and many others. With the advent of metamaterials, (nonlinear) optical properties can nowadays be tuned, controlled, and designed, allowing for the exploration of new physics. In this article, the fluctuations of the electromagnetic field in the presence of such materials are investigated.

Fluctuational electrodynamics, combining classical electrodynamics with quantum and thermal noise, is a powerful framework to study effects which appear in equilibrium (such as Casimir forces) as well as those found out of equilibrium (such as radiative heat transfer). So far, this concept relies on the optical linearity of the involved objects.

In this article, fluctuational electrodynamics is adapted to describe also objects with nonlinear optical response, including the amendment of the noise (so called Rytov currents). Most notably, electric currents fluctuating because of noise and induced currents due to incident waves become coupled, giving rise to new phenomena. As an example, the Casimir force between two plates with nonlinear optical properties is computed, which has a different distance dependence at close proximity compared to the linear case.

H. Soo and M. Krüger, Fluctuational electrodynamics for nonlinear media, EPL 115, 41002 (2016)