Temperature-dependent van der Waals atom-surface interaction

The van der Waals (vW) long-range atom-surface interaction is often described as a dipole-dipole interaction, originating in the coupling between the quantum dipole fluctuations of an atom and the instantaneously induced fluctuations in the electrostatic image. Due to the finite light velocity, this atom-surface interaction is better understood in the more general frame of the Casimir-Polder theory. Recently, it was demonstrated that the Casimir-Polder interaction undergoes an observable dependence on the vacuum temperature. In a shorter distance range, it is expected that thermally excited surface modes could resonantly modify the vW interaction, when it coincides with a virtual atomic absorption. This search for a temperature dependence motivates this experimental work, in which the vW interaction exerted onto the Cs (8P) level at a typical distance below 100 nm through selective reflection on Cs vapour is analyzed. For materials such as CaF₂ and BaF₂, surface resonances are nearly coincident with the strong dipole (absorptive) couplings to Cs(7D) . The present status of the measurements seem to indicate for CaF₂ a vW interaction much stronger than expected. This intringuing result leads to secure quantitative predictions, notably concerning the spectroscopy of surface resonances, and their temperature dependence. Reflectance spectra of CaF₂ or BaF₂, and sapphire as well, were recorded with an interferometer as a function of temperature, in a range spanning from room temperature to 500AdegC. Through a semi-quantum model, allowing for multi-phonon absorption that are of importance in the far infrared range, the surface response could be evaluated with a reasonable insensitivity to the choice of the modelling. It appears that for CaF₂ and Cs(8P), the temperature dependence of the vW interaction is mostly related to quantum effects of the thermal excitation, while for BaF₂, the details of te- mperature shift and broadening of the crystal resonances mix-up with the fundamental quantum effects, as due to a stronger resonance between BaF₂ and Cs(8P) emission.