Resonant van der Waals Repulsion between Excited Cs Atoms and Sapphire Surface

Abstract

We explore a situation where the van der Waals long-range atom-surface interaction is repulsive. This repulsion originates in a resonant coupling between a virtual emission at $12.15\ensuremath{\mu}\mathrm{m}$ of a ${\mathrm{Cs}}^{*}({6D}_{3/2})$ atom and a virtual excitation of a surface polariton in sapphire. The experimental evidence is based upon the analysis of the spectroscopic response of Cs* in the near-infrared range with a technique that probes a distance range $\ensuremath{\sim}100\mathrm{nm}$ away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation.