Frequency modulations due to domain dynamics in terahertz quantum cascade lasers

The light output of quantum cascade lasers is strongly affected by electric field-domain oscillations if the laser shows electrical instabilities. This can result in a substantial broadening of the emission spectrum, which is investigated here by detailed simulations for a terahertz device. We explain how the light pulsation is affected by electrical injection conditions and external capacitances. Experimental data confirm that the oscillation frequencies are not essentially affected by circuit conditions, while a large external capacitance produces almost non-observable oscillations due to stabilization of the circuit.