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Sub-Doppler Spectroscopy in a confined vapour: towards three-dimensional confinement.

Elias Moufarej, Philippe Ballin, Athanasios Laliotis, Isabelle Maurin, Daniel Bloch, S. Villalba, L. Lenci, S. Barreiro, A. Lezama, H. Failache

Published Sep 22, 2014• 0 citations
OpenAlex

Abstract

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 .

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Physical Review A•May 1, 1999•87 citations
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.
Artificial thin glass opals can be infiltrated with a resonant alkali-metal vapour, providing novel types of hybrid systems. The reflection at the interface between the substrate and the opal yields a resonant signal, which exhibits sub-Doppler structures in linear spectroscopy for a range of oblique incidences. This result is suspected to originate in an effect of the three-dimensional confinement of the vapour in the opal interstices. It is here extended to a situation where the opal is limited to a few- or even a single-layer opal film, which is a kind of bidimensional grating. We have developed a flexible one-dimensional layered optical model, well suited for a Langmuir-Blodgett opal. Once extended to the case of a resonant infiltration, the model reproduces quick variations of the lineshape with incidence angle or polarization. Alternately, for an opal limited to a single layer of identical spheres, a three-dimensional numerical calculation was developed. It predicts crystalline anisotropy, which is demonstrated through diffraction on an empty opal made of a single layer of polystyrene spheres.
We report on the spectroscopy in a resonant vapor film, in a regime in which the inhomogeneous transit time induces a velocity selection. For cell thickness (10--1000 \ensuremath{\mu}m) smaller than the transverse dimensions and for low enough atomic density, the atomic trajectories are essentially free flights from wall to wall. Hence, a single laser beam irradiation is sufficient to observe sub-Doppler resonances because of the transient atomic response, which enhances the response of the slowest atoms flying nearly parallel to the wall. The peculiar features of this velocity selection are presented in the linear regime as well as in the case of saturation induced by the optical pumping. Also, we report, in the case of the Cs D2{D}_{2}D2​ line (852 nm), on the passage from a two-level linear-response regime to an optical pumping regime when increasing the incident intensity. We also describe the influence of several other parameters, including cell thickness and laser beam size and shape, on the experimental line shapes. An extension to Doppler-free polarization spectroscopy in a thin cell with a single beam, and to saturated absorption, is also reported.