Pulse propagation phenomena are central to ultrashort pulse generation and amplification in lasers~(1-5). In the coherent regime, the phase relationship between the pulse and the material transition is preserved, allowing both optical fields and material states to be controlled~6. The most prominent form of coherent manipulation is Rabi flopping~7, a phenomenon well established in few-level absorbers, including atoms and single quantum dots~(8-19). However, Rabi flopping is generally much weaker in semiconductors because of strong dephasing in the electronic bands, in contrast to discrete-level systems. Although low-density induced coherent oscillations have been observed in semiconductor absorbers~(11,13-20), coherent pulse propagation phenomena in active semiconductor devices have not been observed. In this Letter, we explore coherent pulse propagation in an operating quantum cascade laser and directly observe Rabi flopping and coherent pulse reshaping. This work demonstrates the applicability of few-level models for quantum cascade lasers and may stimulate novel approaches to short pulse generation ~(21,22).
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