An Instability Study in Terahertz Quantum Cascade Lasers

Terahertz (1−10 THz, 30−300 μm) frequencies are among the least developed electromagnetic spectra even though they have wide ranging applications in spectroscopy, imaging, and remote sensing [1]. Since the report of quantum-cascade-lasers (QCLs) operating in the terahertz spectral region by Köhler et al. [2] there has been significant progress concerning the available frequencies, the temperature performance, and the understanding of the dynamics of a QCL. [3]. QCLs have a relatively long gain recovery time compared to the cavity round-trip time. Therefore, saturable absorber (SA) dominated self-mode locking seems impossible in typical QCLs. However, the elusive Risken-Nummedal-Graham-Haken (RNGH)-like instability is observed in QCLs [4]. Instability mechanisms in QCLs can be evaluated by linear stability analysis in which the criteria for RNGH instability is expressed in terms of the parametric gain g(Ω) as a function of the resonance frequency Ω: