Femtosecond Infrared Emission Resulting from Coherent Charge Oscillations in Quantum Wells

We excite quantum beats in a quantum well using 12 fs optical pulses. The resulting coherent charge oscillation, in the tens-of-tetrahertz range, radiates a corresponding infrared electromagnetic wave which is monitored through heterodyne detection using as a reference a nearly single-cycle infrared pulse. The direct observation of this wave-packet oscillation yields information on the relaxation of intersubband coherence in quantum wells. Furthermore, the experiment yields simultaneously the femtosecond dynamics of both diagonal and off-diagonal matrix elements of the density operator.