Recently, on-chip quantum-cascade-laser-based frequency combs are gaining increasing attention both in the Mid-IR and in the THz spectral regions. THz devices offer the possibility of filling the gap of comb sources in a spectral region were no table-top comb is available. I will discuss direct THz comb generation from both homogeneous and heterogeneous quantum cascade lasers. Octave spanning emission spectra and comb operation on bandwidth larger than 1 THz are reported for heterogeneous cascades. I will also report on a series of new structures with homogeneous cascade design that feature a very low threshold current density (< 100 A/cm2), a bandwidth of roughly 1 THz centered a 3 THz and an extremely wide bandwidth (>1.8 THz) when driven in the NDR region. This extremely broadband emission in the NDR is studied as well with NEGF simulation and is based on an interplay between strong photon assisted transport due to the highly diagonal transition and domain formation.These structures are also showing RF injection locking with extremely reduced microwave powers. We will discuss locking experiments as well as a method to finely control the repetition rate of the laser based on controlled optical feedback.
Time resolved spectral measurements aimed to clarify the physics of field domains in the NDR will be also presented.