Three-dimensional confinement of vapor in nanostructures for sub-Doppler optical resolution

International audienceWe 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 line