title: Dynamics of ultra-broadband terahertz quantum cascade lasers for comb operation.
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 direct RF modulation. Depending on the pump current we observe different regimes of operation, where RF spectra displaying single and multiple narrow beat-note signals alternate with spectra showing a single beat-note characterized by an intense phase-noise, extending over a bandwidth up to a few GHz. We investigate the relation between this 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 strong broadening of the THz emission spectrum.
authors.forename: Hua
authors.initials: H
authors.lastname: Li
authors.forename: Pierre
authors.initials: P
authors.lastname: Laffaille
authors.forename: Djamal
authors.initials: D
authors.lastname: Gacemi
authors.forename: Marc
authors.initials: M
authors.lastname: Apfel
authors.forename: Carlo
authors.initials: C
authors.lastname: Sirtori
authors.forename: Jeremie
authors.initials: J
authors.lastname: Leonardon
authors.forename: Giorgio
authors.initials: G
authors.lastname: Santarelli
authors.forename: Markus
authors.initials: M
authors.lastname: Rösch
authors.forename: Giacomo
authors.initials: G
authors.lastname: Scalari
authors.forename: Mattias
authors.initials: M
authors.lastname: Beck
authors.forename: Jerome
authors.initials: J
authors.lastname: Faist
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Hänsel
authors.forename: Ronald
authors.initials: R
authors.lastname: Holzwarth
authors.forename: Stefano
authors.initials: S
authors.lastname: Barbieri