Switching on and off the Risken-Nummedal-Graham-Haken instability in quantum cascade lasers

We report on experimental observation of low-threshold Risken-Nummedal-Graham-Haken (RNGH) instability in mid-infrared InGaAs/InAlAs Quantum Cascade Lasers (QCLs) operating at 8 $\mu$m wavelength. The devices employ sequential resonant tunneling and can provide optical gain on the diagonal or vertical transition. Depending on the tunneling resonance detuning, the lasers can operate either on a stable part of I-V curve or with unstable electric field domain formation. We report how all these features impact the occurrence of the RNGH instability with broadband multimode emission, and show how they can be tailored with an applied bias or via optical excitation of free carriers. We present the results of 2nd order interferometric autocorrelation measurements and time-resolved spectral measurements in free-running QCLs and QCLs subjected to optical pumping. We explain the diversity of dynamic regimes in QCLs using a simple analytical expression for the second threshold.