Vapour confined in an opal of nanospheres: Observation of sub-Doppler optical resonances reminiscent of the Dicke narrowing

Atomic vapours provide convenient frequency references, made precise when insensitive to the Doppler broadening, and there is a growing technological need to fabricate compact atomic clocks. Various technological reasons, including chemical and adsorption processes, may limit the confinement of a vapour, but an intrinsic limit lies in the mesoscopic nature of a confined gas, when the walls confine the atomic motion.