Dynamics of THz quantum cascade lasers under external optical feedback

The investigation of quantum cascade laser (QCL) dynamics under external optical feedback is crucial for the development of more precise QCL-based sensing and communication devices. In this theoretical contribution, we present our recent numerical results of THz frequency exemplary QCL designs [1] in which we observe different dynamical regimes arising in an external optical feedback configuration [2]. In particular, we identify transitions from single to multimode operation, appearance of frequency combs, and mixed states depending on the length of the external cavity. The parameters used in the dynamics simulations are extracted from carrier transport simulations modelled by a density matrix approach presented in [3], which uses the first neighbour and tight binding approximations, together with the infinite period QCL consideration. The relevant scattering mechanisms are described as perturbations with Fermi's golden rule, using the wavefunctions and material parameters as inputs. For modelling of the laser dynamics under external optical feedback, two approaches based on Maxwell-Bloch equations are used: (i) effective semiconductor MaxwellBloch equations [4] with current-dependent input parameters and (ii) standard Maxwell-Bloch equations in the rotating wave approximation (RWA) and the slowly varying envelope approximation (SVEA).