Modeling self-starting harmonic mode locking in terahertz quantum cascade lasers

Over the last decade, quantum cascade lasers (QCLs) have become established sources of electromagnetic radiation in the terahertz domain and promise several interesting applications, such as time-resolved THz spectroscopy [1]. The ultrashort pulses which are required for this application can be generated by mode locking. While active mode locking (AML) has been demonstrated for QCLs by several research groups, passive mode locking (PML) has not been realized yet, with a potentially limiting factor being the fast gain recovery mechanism in QCLs [2]. There is strong interest in PML as it requires less external circuitry which would enable more compact pulse generators. Recent theoretical work focused on the possibility of passively mode locked QCLs [2]. Here, an absorber region was placed in the middle of two gain regions and the complete structure was modeled using the Maxwell-Bloch equations [3]. The simulation results showed a colliding pulse mode locking (CPML) scenario with two pulses per round trip, where the effective round trip time was halved. Thus, the gain recovery mechanism became slower compared to the effective round trip time and the formation of stable pulses was enabled.