Direct Measurement of HH2-HH1 Intersubband Lifetimes in SiGe Quantum Cascade Structures

We report the first direct measurement of the HH2-HH1 relaxation time for a SiGe QW with transition energy above the optical phonon frequency. The experiments have been performed for an active, electrically biased SiGe structure, and thus our results constitute a key parameter for the design and dynamic simulation of SiGe quantum cascade laser structures. The paper presents the 'as-measured' photocurrent signal data, from which only a virtually constant background has been subtracted for the 'TM first' and 'TE first' cases. On the timescale of our experiment, an exponential HH2-HH1 decay with a single relaxation time significantly larger (~550 fs) than the time resolution of the experiment is observed when the sample is excited by the TM pulse first. In addition the technique provides a path to the measurement of dynamics in quantum cascade structures under operating electrical injection conditions