PACS. 42.50.Gy -Effects of atomic coherence on propagation, absorption, and amplification of light; electromagnetically induced transparency and absorption. PACS. 32.70.Jz -Line shapes, widths, and shifts. PACS. 42.50.Md -Optical transient phenomena: quantum beats, photon echo, free-induction decay, dephasings and revivals, optical nutation, and self-induced transparency.Abstract.-In a thin cell of dilute vapour, the absorption spectrum exhibits sub-Doppler features due to the relative enhancement of the slow atom contribution, with respect to the transient nature of the interaction with moving atoms. For a two-level system in the linear regime, the narrowest response is predicted to be found for a λ/2 thickness, as an effect of the coherent character of the dipole response as early described by Romer and Dicke (Phys. Rev., 99 (1955) 532) in the microwave regime. We report here on the direct observation of this effect in the optical regime in an ultra-thin vapour cell. This effect is shown to vanish for a thickness equal to λ, and a revival is observed at 3λ/2, as expected from the predicted λ-periodicity. The experiment is performed on the D1 resonance line of Cs vapour (λ = 894 nm), in a specially designed cell, whose thickness varies locally.

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Since the recent realization of extremely thin vapour cells (local thickness: 20-1000 nm), we investigate the optical properties of these 1-D confined vapours. Aside from their interest for Doppler-free spectroscopy, nanocells offer a new tool to evaluate collisional shift and broadening, yielding an access to the open problem of collisions under confinement. It also allows probing of the atom-surface interaction in a range of unusual short distances. The experimental exploration of the distance dependence, normally evolving according to the z-3 van der Waals (vW) dependence (z : the atom-surface distance), is worth doing because it could be affected by imperfections of the real surface, such as roughness, adsorbed impurities or charges. A detailed lineshape analysis is now under progress, with tight constraints imposed to the fitting by the twin information brought by simultaneous reflection and transmission spectra. Another issue is a possible resonant enhancement, susceptible to induce a repulsive vW, due to the coupling between atom excitation and a surface mode.

This paper presents our current measurements in a vapor nanocell aiming at a test of the distance-dependence of the atom-surface interaction, when simple asymptotic descriptions may turn to be not valid. A state-of-the-art of atomsurface interaction measurements is provided as an introduction, along with the comparison with the theory of the van der Waals (or Casimir-Polder) interaction; it is followed by a presentation of the most salient features of nanocell spectroscopy.

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...

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Transmission spectroscopy in an extremely-thin vapor cell is demonstrated where the local thickness is in the 20-100 nm range. The van der Waals atom-surface interaction can hence be probed in an unexplored distance range.

The recent fabrication of "nanocells" of vapour [1], with local cell thickness typically spanning from 20 nm to 1 m, has opened a realm of novel prospects, with the notable possibility of detecting atom-surface interaction effects in an unexplored range of distances.In spite of the ubiquity of the long-range atom-surface van der Waals (vW) interaction, that can be viewed as the dipole-dipole interaction between the quantum dipole fluctuations of an atom and its electric image, the choice of methods to investigate this interaction is actually scarce. Most often, these methods rely on mechanical effects -even for laser cooled atoms-and are applicable only to ground state atoms or long-lived states. This explains that the only accurate measurements of the Z-3 distance law of the interaction -for z: the atom-surface distance-, that should normally applies in a typical 1000-1 nm range, have been limited to the 3000-500 nm range, with Rydberg, atoms that are strongly interacting, with energy shifts up to 200 MHz, and are long-lived, allowing for a detection that is partly mechanical [2I. Spectroscopy in a nanocell should extend the possibility to probe short-lived excited states, typical of optical methods such as Selective Reflection (SR) spectroscopy, developed for a long time in our group to probe the VW interaction. The advantage of nanocell spectroscopy is that it yields an access to an adjustable range of short atomsurface distance, while the probed depth in SR spectroscopy is limited to X/2n.We have started to investigate in details the strong vW shift induced on high-lying excited states, such as Cs (6D), that is probed at 917nm (6D51) or 921 nm (6D312) after a prior excitation on the Cs Do line 6S1/2-6P312 at 852 nm. Our analysis currently encompass a 40-130 nm range of thickness, leading to the observation of frequency shifts -relatively to the transition frequency in the free-space-as large as 10 GHz. The van der Waals potential between two walls is predicted to obey a transcendental Lerch function, as a result of the induced multiple "electric images", and to follow a L3 dependence (L: the local thickness). Such an approximate L3 dependence has been verified for the observed shift (relatively to the free-space resonance) affecting the peak of the transmission lineshape. However, as already known from SR spectroscopy, the apparent shift is not by itself a sufficient criterion for the measurement of the vW interaction. Rather, a consistent lineshape analysis is required for a determination of the coupling, strength of the vW interaction. We report here our preliminary results, providing such an analysis for the first time in this range of small distances.Our systematic analysis includes a monitoring of the pressure effects, and of the saturation effects induced by the pumping beam. It now permits to sort lineshapes for which these surface-independent effects are negligible. Also, because the design of a nanocell implies an excellent parallelism of the internal windows, responsible for an intrinsic Faby-Perot ...