Coherent control of a THz-intersubband polarization in a 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 THz pulse excites a macroscopic polarization between the quantum well first and second subband, a second, temporally delayed, control pulse switches the polarization off or refreshes it depending on the relative time delay between the pulses. The switching is directly demonstrated in the time-domain for the few picosecond long free-induction decay of the induced polarization. Model calculations based on the optical Bloch equations agree well with the experimental data.