Signature of collective effects in the frequency-comb-induced radiation pressure force

In recent years, the applications of optical frequency combs (FCs) have expanded to laser cooling and trapping of atoms and ions, and quantum communication. For these novel and intriguing applications of FCs, it is necessary to understand the FC light-matter interactions, and to verify whether and to what extent they are comparable to continuous-wave (cw) light-matter interactions. It is particularly important to understand FC light scattering and accompanying effects.

We investigate the modifications of the frequency-comb-induced radiation pressure force on cold ⁸⁷Rb atoms that are induced by the collective effects. Collective effects include both coherent and incoherent contributions and depend on the optical thickness of the atomic cloud. We observe reduction and broadening of the comb-induced force when the cloud's optical thickness increases, and compare the measured results with predictions of a coherent model based on the timed Dicke state approach and an incoherent scattering model based on the shadow effect explained by the Beer-Lambert law. Both models describe the experimental results well, indicating that an incoherent scattering approach is sufficient to explain the observed modifications of the comb-induced force, even for larger optical thicknesses. The results support the analogy between the frequency comb and continuous-wave laser-atom interaction and thus pave the way toward novel frequency comb applications in laser cooling and quantum communication.