Observation of large atomic-recoil-induced asymmetries in cold atom spectroscopy

The atomic-recoil effect leads to large (25%) asymmetries in a nearly ideal form of saturation spectroscopy based on Ca atoms that have been laser-cooled to 10 μK. Starting with spectra from the more familiar Doppler-broadened domain, we show how the fundamental asymmetry between absorption and stimulated emission of light manifests itself when shorter spectroscopic pulses lead to the Fourier transform regime. These effects occur on frequency scales much larger than the size of the recoil shift itself, and have not been described before in saturation spectroscopy. These results directly impact precision spectroscopic measurements.