Coherent control of a THz intersubband polarization in a voltage controlled single quantum well

Ultrashort terahertz pulses in the far-infrared spectral region centered around 2 terahertz are used to coherently control an intersubband polarization in a GaAs/AlGaAs quantum well structure at low temperature. While the first pulse excites a macroscopic polarization, a second, temporally delayed pulse switches the polarization off or refreshes it depending on the relative time delay. The switching is directly demonstrated in the time-domain for the few picosecond long free-induction decay of the induced polarization. Additionally, the absorption line is found to be purely homogeneously broadened. Model calculations based on the optical Bloch equations agree well with the experimental data.