Time-domain spectroscopy of THz quantum cascade lasers: Theoretical and experimental aspects

We present fully developed model of the time-domain spectroscopy of the terahertz gain medium. Based on this model we simulated pulse amplification and the coherent response of the gain medium. We show that except of generally expected amplification of the pulse after interaction with the gain medium, other effects like the spectral shaping, spectral ´hole burning´, and the phase coherence revival typical for the pump/probe technique are directly observable in the time domain signal. This unique measurement technique is further discussed with respect to the probing terahertz quantum cascade structures and we draw the theoretical limits of them to amplify the broadband (ps long) pulses.