Sub-micrometric vapor cell : a new tool for spectroscopy and atom-surface interaction

Abstract

With the recent development of extremely thin vapor cells (ETC), whose thickness varies locally in a typical range 50 nm-1000 nm [l], we have embarked in the exploration of the prospects offered by this new tool for laser spectroscopy and studies of the atom-surface interaction.In the last years, we had demonstrated a novel principle of sub-Doppler spectroscopy based on the fact, that in a short column of dilute vapor, the atoms fly wall-to wall and exhibit an anisotropic distribution of free flight. Indeed, for an irradiation under normal incidence, the transient response of the slower atoms is enhanced. Such a method had already been applied to a variety of situations (velocity-dependent optical pumping, linear absorption, two-photon transition, ..,). However, up to the advent of ETC, the cell thickness was much larger than the optical wavelength h, and the sub-Doppler signature was essentially a marginal contribution to the overall signal, usually observable only through the trick of a FM (frequency-modulation) method.In the linear two-level spectroscopy, the optimal situation is obtained when the thickness is h/2, a situation that maximizes the contrast between the coherent summing of the transient response and the Doppler-broadened background. Conversely, at a h thickness, the narrow part is washed out, as early mentioned by Dicke in the 50's for an equivalent microwave situation. A partial revival of the coherent peak is obtained for a 3h/2 thickness; these oscillations in the transient behavior, with their characteristic h pseudo-periodicity, should be distinguished from the Fabry-Ptrot behavior of an ETC -as due to the intrinsic parallelism of the windows-, that is also observed, and whose periodicity is h/2. A systematic study, in transmission and reflection, was performed on the isolated h.f.s. component of the D, line of Cs. For further applications, it should be noted that this observation of a sub-Doppler narrowing, optimal at h/2, is performed in a linear spectroscopy approach, intrinsically applicable to any atomic or molecular line.For an atom at a short distance from a surface, the energy of transitions is shifted under the influence of the longrange van der Waals (vW) interaction. However, with its dependence (z : atom-surface distance), this interaction yields a current spectroscopic signature for distances << lpm, a range that could not be reached in the early developments of thin vapor cell spectroscopy. With an ETC, the spatial dependence of the interaction can be solved, and in the experiments, a considerable lineshape distortion and shift -largely exceeding the equivalent phenomena in selective reflection spectroscopy-, has been observed on the Cs DI resonance line, when the local thickness of the cell is very small (50-100 nm). The observations are in a fair agreement with the theoretical modelling. Experiments in progress notably deal with excited Cs levels that can resonantly couple, in a near-field atom-surface interaction, with surface-polariton modes, like Cs (6D3p) with sapphire, th...