Real-time multimode dynamics of terahertz quantum cascade lasers via intracavity self-detection: observation of self mode-locked population pulsations

• DOI: 10.1364/oe.444295
• Publication Date: 2021-12-23T15:31:30Z
• Authors: Hua Li; Wenjian Wan; Ziping Li; J. C. Cao; Sylvie Lepillet; Jean-Francois Lampin; Kevin Froberger; Lorenzo Columbo; Massimo Brambilla; Stefano Barbieri
• Keywords: Population inversion; Modulation (music); Oscillation (cell signaling); Mode-Locking

Mode-locking operation and multimode instabilities in Terahertz (THz) quantum cascade lasers (QCLs) have been intensively investigated during the last decade. These studies unveiled a rich phenomenology, owing to unique properties of these lasers, particular their ultrafast gain medium. Thanks this, QCLs modulation intracavity field intensity gives rise strong population inversion, directly affecting laser current. In this work we show that property can be used study real-time dynamics THz QCLs, using self-detection technique combined with 60GHz oscilloscope. To demonstrate potential investigate 4.2THz QCL operating free-running, observe self-starting periodic current, producing trains regularly spaced, ∼100ps-long pulses. Depending on drive current find two distinct regimes oscillation dramatically different properties: first regime at fundamental cavity repetition rate, characterised by large amplitude phase noise, coherence times few tens periods; much more regular second-harmonic-comb regime, typical ∼105 periods. We interpret measurements set effective semiconductor Maxwell-Bloch equations qualitatively reproduce features dynamics, indicating observed carrier-density optical pulses are antiphase, appear as rather shallow top continuous wave background. its simple implementation versatility, demonstrated broadband is powerful tool for QCLs.