Terahertz time domain spectroscopy of phonon-depopulation based quantum cascade lasers

The gain and loss in a terahertz quantum cascade laser based on a longitudinal optical phonon depopulation scheme is studied using terahertz time domain spectroscopy. At laser threshold the gain is found to be clamped at 25 cm(-1) for a 1 mm long device and the full width at half maximum of the spectral gain during laser action is 0.6 THz. At low biases, before laser action, significant absorption features are observed for frequencies lower and higher than the laser emission frequency. We assign their onset to the lower laser level being populated from a parasitic electronic channel. The absorption transitions are identified from bandstructure simulations and from the oscillator strengths of the subband transitions as a function of the applied field.