High-resolution roughness measurements using cold atom reflection

Abstract

Evanescent wave atomic mirrors have been shown to be very sensitive to the surface roughness of the glass substrate on which the evanescent wave is formed. Non-specular reflection of slow atoms at normal incidence has been observed for surface roughness as low as 0.1 nm. We interpret this behaviour in terms of diffraction of the atomic De Broglie wave on the modulated potential created by the interference of the evanescent wave with light scattered by the rough surface. However these experiments could not separate the specular peak of reflected atoms from the diffuse part. We have developed a new method to measure the velocity distribution of the atoms after the bounce with a much better resolution, using Raman transitions. Two phase locked counterpropagating laser beams induce a Raman transition between the ground state hyperfine levels of the /sup 85/Rb atoms falling from a magneto-optical trap. The resonance condition is only fulfilled for atoms with a transverse velocity controlled by the interaction time between the atoms and the beams. These velocity selected atoms bounce on the mirror and their velocity distribution after reflection can be measured using the same Raman beams, by scanning their difference in frequency.