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 mm of a Cs ء ͑6D 3͞2 ͒ 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 ϳ100 nm away from the sapphire surface. We also demonstrate the critical dependence of atom-surface forces on the sapphire crystal orientation. PACS numbers: 34.50.Dy, 12.20.Fv, 71.36. + c The van der Waals (vW) force between neutral polarizable systems [1,2] represents a universal interaction of paramount importance in numerous fields of physics, chemistry, and biology. Its attractive character is essential for the cohesion of many chemical or biological systems. vW attraction between atomic systems and metallic or dielectric bodies is also a fundamental property in cavity quantum electrodynamics (QED) [3], and its main characteristics have been experimentally studied by means of mechanical approaches (atomic beam deflection produced by surfaces [4,5], energy threshold in atomic mirrors [6]), or, in our group, spectroscopic approaches (spectral monitoring of surface-induced atomic level shifts [7-10]).In the nonretarded regime, the vW interaction between an atom and a surface originates in the quantum fluctuations of the atomic dipole: The fluctuating dipole polarizes the surface, and induces a dipole image instantaneously correlated with the atomic dipole. This near-field image is responsible for the attractive character of the vW interaction, which scales in 1͞z 3 (z is the atom-surface distance) [1].Is it possible to turn this near-field attraction into a repulsion? The answer is yes, if a resonant coupling between the fluctuating atomic dipole and a surface excitation can occur [11][12][13]. For a dispersive dielectric medium with a complex permittivity´͑v͒, the well-known electrostatic image coefficient S ͑´2 1͒͑͞´1 1͒ (with 0 # S # 1) has to be generalized to a complex frequency-dependent surface response [13]:whose resonances ("surface polaritons" [14]) are determined by the poles of S͑v͒.Consider an excited atom for which one of the dipoleallowed deexcitation channels (at frequency v a ) is near resonant with a surface polariton. Because of nonradiative, virtual coupling between the atom and surface (atomic decay followed by surface excitation), the surface response at frequency v a is magnified, implying a dielectric image coefficient, 2Re͓S͑v a ͔͒, possibly larger than one-the value obtained for an ideal reflector. Because of the resonant enhancement, S is complex and the image dipole is dephased as well. When this image is phase reversed, the near-field vW attraction is turned into a near-field vW repulsion, also scaling in 1͞z 3 . This is one among the rare situations of a long-range, state-selective repulsion exerted by a cavity wall on an atom, at distances spanning f...
