In the spirit of our systematic investigation of resonant atomic vapour (Cs vapour) in micro-and nano-cells (1D confinement), we investigate the behaviour of a vapour confined in the nanometric interstitial regions of an opal made of glass spheres (3D confinement). Sub-Doppler lines are observed in preliminary experiments
When a vapour is confined, the collisions to the confinement wall usually cause a broadening of the transitions. However velocity selective processes and the coherent enhancement of some transient contributions can lead to sub-Doppler contributions because of the confinement. In the optical range, this had been demonstrated for a one-dimensional confinement with micro- or nano-cells of a small thickness. A two-dimensional confinement in a hollow fiber had shown sub-Doppler contributions, but only equivalent to macrosocpic non linear saturated absorption spectroscopy. Here, we describe experiments showing that a thermal vapor confined in a three-dimensional nanostructure can exhibit sub-Doppler features in a linear regime. We perform reflection spectroscopy on a Cs vapour cell whose window is covered with a thin opal film (typically, 10 or 20 layers of ~ 1µm or 400 nm diameter spheres). We demonstrate, both experimentally and through a modeling that the sub-Doppler structures observed for a large range of oblique incidence angles (~ 30-50°) originate from the interstitial regions of the opal, hence corresponding to a three-dimensional submicrometric confinement. We also show that the partially porous nature of the glass spheres allows some LIAD effects. In a different set of experiments, an atomic vapour is confined in glass medium with large size pores (tens of µm) randomly distributed. The effect of scattered light and fluorescence have been distinguished. Also, sub-Doppler features evolving non linearly with the irradiating intensity are observed, originating in a saturated absorption process induced by the backscattered light .
We show that a thermal vapor confined in a nanostructure is of spectroscopic interest. We perform reflection spectroscopy on a Cs vapour cell whose window is covered with a thin opal film (typically, 10 or 20 layers of ~ 1μm diameter spheres). Sub-Doppler structures appear in the optical spectrum in a purely linear regime of optical excitation and the signal is shown to originate from the interstitial regions of the opal. These narrow spectral structures, observable for a large range of oblique incidence angles (~ 30-50°), are an original feature associated to the 3-D vapor confinement. It remembers a Dicke narrowing, i.e. a Doppler broadening suppression when the atomic motion is sub-wavelength confined. This narrowing, commonly observed in the r.f. domain when a buffer gas ensures a collision confinement effect, had remained elusive in the optical frequency. Also, we describe preliminary experiments performed in a pump-probe technique, intended to elucidate the spatial origin of the narrow contribution. We finally discuss how our results allow envisioning micron-size references for optical frequency clocks, and high resolution spectroscopy of weak and hard-to-saturate molecular lines.
We confine a Cs thermal vapor in the interstitial regions of a glass opal. We perform linear reflection spectroscopy on a cell whose window is covered with a thin film (10 or 20 layers) of ∼1000 nm (or 400 nm) diameter glass spheres and observe sub-Doppler structures in the optical spectrum for a large range of oblique incidences. This original feature associated with the inner (3-dimensional) confinement of the vapor in the interstitial regions of the opal evokes a Dicke narrowing. We finally consider possible micron-size references for optical frequency clocks based on weak, hard to saturate, molecular lines.
Ce mémoire rapporte diverses expériences de spectroscopie visant à étudier le comportement d une vapeur atomique soumise à un confinement tridimensionnel sub-micrométrique. Nous présentons notamment les spectres expérimentaux de réflexion (raies D1 et D2) réalisées sur des cellules macroscopiques contenant une vapeur de césium et une opale constituée d un nombre contrôlé de couches de nano-billes de silice (déposé par la technique Langmuir-Blodgett). En fonction de l angle incidence du faisceau illuminant l opale, deux types de signaux sub-Doppler sont observés via une détection par modulation de fréquence : l un observable à des angles d incidence faibles, rappelant les formes de raies habituelles d une réflexion sélective sur une interface plane de verre, et l autre, superposé à un signal de fond large, pour des angles d incidence importants. Cette structure, inédite pour des expériences de réflexion sélective, peut être isolée par une expérience pompe-sonde ou une détection 2f. Elle pourrait être la signature d un rétrécissement Dicke, conséquence du confinement des atomes sur une distance équivalente à une fraction de longueur d onde.