Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser

The dynamics of optical gain in an InGaAs/AlInAs quantum cascade laser is studied in midinfrared pump-probe measurements. Pump and probe pulses of identical wavelength, polarization, and propagation direction through the laser waveguide are detected separately by electro-optic sampling. For injection currents below the lasing threshold, we observe an absorption decrease in the laser transition with a ≈3 ps recovery time. Above threshold, the gain strongly saturates and shows a fast recovery time decreasing with current. Such kinetics is superimposed by oscillations with a frequency of 0.8 THz originating from coherent electron tunneling through the injection barrier.

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