摘要: Mode-locking operation and multimode instabilities in Terahertz (THz) quantum
cascade lasers (QCLs) have been intensively investigated during the last
decade. These studies have unveiled a rich phenomenology, owing to the unique
properties of these lasers, in particular their ultrafast gain medium. Thanks
to this, in QCLs a modulation of the intracavity field intensity gives rise to
a strong modulation of the population inversion, directly affecting the laser
current. In this work we show that this property can be used to study the
real-time dynamics of multimode THz QCLs, using a self-detection technique
combined with a 60GHz real-time oscilloscope. To demonstrate the potential of
this technique we investigate a free-running 4.2THz QCL, and observe a
self-starting periodic modulation of the laser current, producing trains of
regularly spaced, ~100ps-long pulses. Depending on the drive current we find
two regimes of oscillation with dramatically different properties: a first
regime at the fundamental repetition rate, characterised by large amplitude and
phase noise, with coherence times of a few tens of periods; a much more regular
second-harmonic-comb regime, with typical coherence times of ~105 oscillation
periods. We interpret these measurements using a set of effective semiconductor
Maxwell-Bloch equations that qualitatively reproduce the fundamental features
of the laser dynamics, indicating that the observed carrier-density and optical
pulses are in antiphase, and appear as a rather shallow modulation on top of a
continuous wave background. Thanks to its simplicity and versatility, the
demonstrated technique is a powerful tool for the study of ultrafast dynamics
in THz QCLs.
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