Casimir-Polder force for a polarizable molecule near a dielectric substrate out of thermal equilibrium

TL;DR

This paper explores how the Casimir-Polder force on a molecule near a dielectric substrate out of thermal equilibrium can be manipulated, leading to attractive-to-repulsive transitions influenced by temperature differences and molecular properties.

Contribution

It reveals novel thermal non-equilibrium behaviors of the Casimir-Polder force, including force transitions at lower temperatures and specific conditions for different molecular polarizations.

Findings

Force transitions depend on temperature differences between substrate and environment.

Transitions can occur at room temperature for certain dielectric substrates.

Manipulation of force direction is possible by varying thermal conditions.

Abstract

We demonstrate that the Casimir-Polder force for a molecule near the surface of a real dielectric substrate out of thermal equilibrium displays distinctive behaviors as compared to that at thermal equilibrium. In particular, when the molecule-substrate separation is much less than the molecular transition wave-length, the CP force in the high temperature limit can be dramatically manipulated by varying the relative magnitude of the temperatures of the substrate and the environment so that the attractive-to-repulsive transition can occur beyond certain a threshold temperature of either the substrate or the environment depending on which one is higher for molecules both in the ground and excited states. More remarkably, when the separation is comparable to the wave-length, such transitions which are impossible at thermal equilibrium may happen for longitudinally polarizable molecules with a small permittivity, while for isotropically polarizable ones the transitions can even occur at room temperature for some dielectric substrates such as sapphire and graphite which is much lower than the temperature for the transition to take place in the thermal equilibrium case, thus making the experimental demonstration of such force transitions easier.