Low-Threshold RNGH Instabilities in Quantum Cascade Lasers

We show that low-threshold Risken-Nummedal-Graham-Haken (RNGH) instabilities in quantum cascade lasers (QCLs) occur due to a combined effect of the carrier coherence and population grating induced in the gain medium. QCLs with a few mm long cavity exhibit intermittent RNGH self-pulsations while regular self-pulsations are possible in short-cavity QCLs, with the cavity length of 100 mu m or smaller. Transient behavior to RNGH self-pulsations in short-cavity QCLs shows features resembling cooperative superradiance which opens a practical way of achieving ultrashort pulse production regimes in the mid-infrared spectral range.