Macroscopic transient absorption in a V-type three-level system

Despite the wide employment of transient absorption spectroscopy in capturing fast-evolving dynamics, the experiments lack further exploration of macroscopic effects beyond the single-atom response. Here, the absorbance of Rb atoms excited and coupled by two near-infrared pulses with a variable interpulse delay is studied both experimentally and theoretically, from the limit of a dilute-gas medium into high-density cases. Our results reveal that with increasing atomic density, the absorption lines are broadened with substructures emerging therein. However, the spectral evolutions are rather distinctive for positive and negative time delays, pointing toward different mechanisms behind them. As a distinguishing characteristic of the V-type levels, delay-dependent oscillations are entwined and complicate the resulting profiles considerably. The obtained intensity- and density-controlled line shapes are well reproduced based on the density matrix theory with the inclusion of pulse propagation. We believe these observations will not only play a crucial role in studying strong-field dynamics in dense systems, but could also serve as key ingredients for optical pulse shaping. (paper)