Harmonic and\nSubharmonic RF Injection Locking of THz\nMetasurface Quantum-Cascade VECSEL

Harmonic and subharmonic RF injection locking is demonstrated\nin\na terahertz (THz) quantum-cascade vertical-external-cavity surface-emitting\nlaser (QC-VECSEL). By tuning the RF injection frequency around integer\nmultiples and submultiples of the cavity round-trip frequency, different\nharmonic and subharmonic orders can be excited in the same device.\nModulation-dependent behavior of the device has been studied with\nrecorded lasing spectral broadening and locking bandwidths in each\ncase. In particular, harmonic injection locking results in the observation\nof harmonic spectra with bandwidths over 200 GHz. A semiclassical\nMaxwell-density matrix formalism has been applied to interpret QC-VECSEL\ndynamics, which aligns well with experimental observations.