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arXiv:1312.2599 · Physical Review A

Sub-Doppler resonances in the backscattered light from random porous media infused with Rb vapor

S. Villalba, Athanasios Laliotis, L. Lenci, Daniel Bloch, A. Lezama, H. Failache

Published Feb 19, 2014• 8 citations
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Abstract

We report on the observation of sub-Doppler resonances on the backscattered light from a random porous glass medium with rubidium vapor filling its interstices. The sub-Doppler spectral lines are the consequence of saturated absorption where the incident laser beam saturates the atomic medium and the backscattered light probes it. Some specificities of the observed spectra reflect the transient atomic evolution under confinement inside the pores. Simplicity, robustness, and potential miniaturization are appealing features of this system as a spectroscopic reference.

DOI
10.1103/physreva.89.023422

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We report on the observation of sub-Doppler resonances on the backscattered light from a random porous glass medium with rubidium vapor filling its interstices. The sub-Doppler spectral lines are the consequence of saturated absorption where the incident laser beam saturates the atomic medium and the backscattered light probes it. Some specificities of the observed spectra reflect the transient atomic evolution under confinement inside the pores. Simplicity, robustness, and potential miniaturization are appealing features of this system as a spectroscopic reference.
We have studied the pump-probe spectroscopy of rubidium vapor confined to the micrometric interstices of a random porous glass. Due to the propagation in the highly scattering medium, the light fields are randomized inside the sample with significant consequences for the atomic spectra. A two-frequency modulation technique was used to isolate the atomic response proportional to the product of the intensities of the pump and probe fields. Unusual line shapes were observed which include relatively narrow structures that are present in spite of the Doppler broadening due to the atomic velocity distribution. A simple theoretical modeling of the light-atom interaction that assumes statistical isotropy of the diffuse light field and mutual temporal incoherence of the pump and probe fields is presented. Using a single adjustable parameter to account for the atomic confinement, the model successfully describes the diversity of the observed spectral line shapes.
We study the two-photon laser excitation to the 5D5/25{D}_{5/2}5D5/2​ energy level of 85Rb^{85}\mathrm{Rb}85Rb atoms contained in the interstices of a porous material made from sintered ground glass with typical pore dimensions in the 10--100 $\ensuremath{\mu}\mathrm{m}$ range. The excitation spectra show unusual flat-top line shapes, which are shown to be the consequence of wave-vector randomization of the laser light in the porous material. For large atomic densities, the spectra are affected by radiation trapping around the D2{D}_{2}D2​ transitions. The effect of the transient atomic response limited by the time of flight between pores walls appears to have a minor influence in the excitation spectra. It is however revealed by the shortening of the temporal evolution of the emitted blue light following a sudden switch-off of the laser excitation.
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 studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of nonradiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given.
We studied spectroscopicaly the fluorescence of a long-lived state of Rubidium for the atomic vapor confined to the micrometric interstices of porous glass cells.The system is studied with the aim of investigating the spectroscopic consequences of the confinement of atoms.The exited level is reached through atom-photon transition.Radiation fields interacting with the atoms in the pores of glass are diffuse due to the propagation in this highly scattering medium.The fluorescence spectra obtained have unusual lineshapes that are well described with a simple theoretical model, considering the diffuse characteristic of the light field.The fluorescence spectra lineshape measured do not exhibit effects due to the three dimensional confinement, however, this effect is noticeable when the fluorescence decay time curves are observed.They show a reduction in its decay time compared with the signal in a cell without confinement.
We studied resonant laser interaction with Rb atoms confined to the interstitial cavities of a random porous glass. Due to diffusive light propagation, the effect of atomic absorption on the light scattered by the sample is almost entirely compensated by atomic fluorescence at low atomic densities. For higher densities, radiation trapping increases the probability of non-radiative decay via atom-wall collisions. A simple connection of the fluorescence/absorption yield to the sample porosity is given.