Transient dynamics and momentum redistribution in cold atoms via recoil-induced resonances

We use an optically dense, anisotropic magneto-optical trap to study recoil-induced resonances (RIRs) in the transient, high-gain regime. We find that two distinct mechanisms govern the atomic dynamics: the finite, frequency-dependent atomic response time, and momentum-space population redistribution. At low input probe intensities, the residual Doppler width of the atoms, combined with the finite atomic response time, result in a linear, transient hysteretic effect that modifies the locations, widths, and magnitudes of the resulting gain spectra depending on the sign of the scan chirp. When larger intensities (\textit{i.e.}, greater than a few $μ$W/cm$^2$) are incident on the atomic sample for several $μ$s, hole-burning in the atomic sample's momentum distribution leads to a coherent population redistribution that persists for approximately 100 $μ$s. We propose using RIRs to engineer the atomic momentum distribution to enhance the nonlinear atom-photon coupling. We present a numerical model, and compare the calculated and experimental results to verify our interpretation.