Terahertz emission from asymmetric, doped quantum wells under resonant pumping

When the intersubband transitions of a doped quantum well are strongly driven by a coherent optical pump, the bare energy levels are split into dressed doublets, a phenomenon known as the ac Stark (or Autler-Townes) effect. In time domain this can be understood as Rabi oscillations of the confined electrons between two subbands. Here we show how, by employing asymmetric quantum wells, the selection rule preventing emission from an upper to a lower state of the same doublet can be lifted. The fluorescence spectrum of such strongly-driven system is then marked by an additional peak that would appear at the Rabi frequency, in the terahertz portion of the electromagnetic spectrum.