  RF injection locking and spectral broadening of a terahertz (THz)
quantum-cascade vertical-external-cavity surface-emitting laser (QC-VECSEL) is
demonstrated. An intra-cryostat VECSEL focusing cavity design is used to enable
continuous-wave lasing with a cavity length over 30 mm which corresponds to a
round-trip frequency near 5 GHz. Strong RF current modulation is injected to
the QC-metasurface electrical bias to pull and lock the round-trip frequency.
The injection locking range at various RF injection powers is recorded and
compared with the injection locking theory. Moreover, the lasing spectrum
broadens from 14 GHz in free-running mode to a maximum spectral width around
100 GHz with 20 dBm of injected RF power. This experimental setup is suitable
for further exploration of active mode-locking and picosecond pulse generation
in THz QC-VECSELs.
