Passive mode-locking of lasers enables a compact way of stable optical pulse generation and is thus of high interest in research and application. Quantum cascade lasers (QCLs) emit radiation in the mid-infrared or terahertz (THz) spectral region and exhibit gain recovery on picosecond timescales. As the cavity round trip time is typically some tens of picoseconds, passively mode-locked operation of QCLs is undoubtedly challenging to achieve. However, short pulses from a THz QCL were recently observed by embedding a graphene saturable absorber into the top contact of the cavity. This contribution presents a model of the dynamic interplay between electric field, gain, and absorber, revealing that pulse formation and stability are possible in a narrow bias range, enhanced by spatial hole burning.
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