  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, we
observe sub-Doppler structures in the optical spectrum for a large range of
oblique incidences. This original feature associated with the inner
(3-dimension) 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.
