Ultrafast THz pulse generation from quantum cascade laser
Feihu WANG (LPA)

In the THz frequency range, a semiconductor based technology platform for
intense and ultrashort pulse generation has yet to be realised. THz
quantum cascade lasers (QCLs) are foundational semiconductor devices for
laser action in the THz range and are suitable medium for pulse
generation. However, to date, the generation of THz pulses from QCLs has
been limited to 10 - 20 ps despite several years of research effort. In
this thesis, I will show how this bottleneck in QCL technology can be
overcome and demonstrate a novel method to realize ultrashort pulses in
the THz range from this compact semiconductor device. Towards this final
objective and realization of my thesis, I will show several milestones
that permitted our goal of short pulses from QCLs to be achieved. Starting
off from the current state-of-the-art, using narrow spectral bandwidth
QCLs in single-plasmon waveguides, I will demonstrate 20 ps long THz
pulses at low temperatures (10K). This is followed by, for the first time,
mode-locking of broad spectral bandwidth QCLs in sub-wavelength
metal-metal waveguides at ‘high’ temperatures (77K). As a result, I will
show 11 ps that are limited by the electrical modulation and the index
dispersion and, importantly, not limited by the inherent spectral
bandwidth. This will then bring us to resolve these limitations and
generate ultrashort pulses (4 ps) with the potential to go considerably
further to the single-cycle regime.