Resonant van der Waals repulsion between excited Cs atoms and sapphire surface

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 mu m of a 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 similar to 100 nm away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation.