  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.
