Abstract

We present an experimental investigation of the multimode dynamics and the coherence of terahertz quantum cascade lasers emitting over a spectral bandwidth of ~1THz. The devices are studied in free-running and under the effect of an external RF modulation. Depending on the drive current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single, broad beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. From the analysis of the emission spectra we show that the narrow beatnotes can result from the beating of THz modes belonging to sub-combs with different offset frequencies. The formation of these sub-combs is ultimately triggered by dispersion, and is the result a rich dynamics involving four-wave mixing and sideband generation through the mutual beating of THz modes. In the broad beatnote regime we investigate the relation between the phase-noise and the dynamics of the THz modes through the electro-optic sampling of the laser emission. We find that when the phase-noise is large, the laser operates in an unstable regime where the lasing modes are incoherent. Under RF modulation of the laser current such instability can be suppressed and the modes coherence recovered, while, simultaneously, generating a significant broadening of the THz emission spectrum.

© 2016 Optical Society of America