Ultrafast probing of light-matter interaction in a midinfrared quantum cascade laser

In this work, we study the interaction of ultrashort midinfrared pulses with the active medium of an In Ga As ∕ In Al As ∕ In P quantum cascade laser emitting at an 11.7 μ m wavelength. Applying an electro-optic sampling technique allowed us to measure the complete phase resolved transmission spectra at operating conditions below and above lasing threshold in a spectral range much broader than the gain band width. Far below threshold, we locate broadband resonant absorption, which spectrally overlaps with the electrically induced gain, forming areas of net absorption and net gain. Above threshold, gain clamping is seen, and it is found that echoes delayed by the round trip time experience spectral pulse shaping converging toward the emission spectrum.