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Time-resolved intensity and spectral changes in a wide-ridge terahertz quantum cascade laser by optical pulse injection

We report the intensity and spectral changes in a multi-mode terahertz quantum cascade laser induced by injecting λ = 800 nm optical pulses through a cavity facet. We find that photogenerated carriers, via modulation of the intersubband gain, increase the threshold current by up to 0.2 A and cause spectral changes such that the individual peaks of the multi-lateral-mode spectra are varied in amplitude with different ratios. It is found that the indirect recombination of electron–hole pairs and thermal relaxation on timescales of ∼700 ns and ∼10 µs, respectively, are involved in the recovery kinetics.