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

摘要

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 [1]. 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. We had previously investigated the 1-D confinement offered by micro- or nano-metric vapour cell [2], evidencing novel Doppler-free resonances, associated either to a velocity-selective process (through a saturation efficient only for slow atoms), either to linear coherent process, reminiscent of a coherent Dicke narrowing initially demonstrated in the radio-frequency domain [3]. However, these sub-Doppler resonances appear only under normal incidence, and the optical beam must exhibit a macroscopic transverse extension. Tighter gas confinements have started to be investigated in various places, with the notable demonstration of all-fibered gas cells [4] (with a ～10 μm diameter), or with gases filling porous media (pore size 10-20 nm range) [4], but gas densities have remained large, hence unsuitable for sub-Doppler spectroscopy. We are now studying dilute vapours (of alkali metal) 3-D confined in the sub-micrometric interstitial regions of an opal of glass nanospheres.