Passive and hybrid mode locking in multi-section terahertz quantum cascade lasers

TL;DR

This paper demonstrates theoretically that stable passive and hybrid mode locking in multi-section terahertz quantum cascade lasers is possible by engineering the cavity and absorber dynamics, enabling few-cycle pulse generation.

Contribution

It introduces a novel multi-section cavity design with strong absorber coupling and fast recovery dynamics to achieve mode locking in QCLs, challenging previous assumptions.

Findings

Stable mode locking achievable in engineered QCLs

Generation of few-cycle terahertz pulses demonstrated

Multiple mode locking approaches explored

Abstract

It is believed that passive mode locking is virtually impossible in quantum cascade lasers (QCLs) because of too fast carrier relaxation time. Here, we revisit this possibility and theoretically show that stable mode locking and pulse durations in the few cycle regime at terahertz (THz) frequencies are possible in suitably engineered bound-to-continuum QCLs. We achieve this by utilizing a multi-section cavity geometry with alternating gain and absorber sections. The critical ingredients are the very strong coupling of the absorber to both field and environment as well as a fast absorber carrier recovery dynamics. Under these conditions, even if the gain relaxation time is several times faster than the cavity round trip time, generation of few-cycle pulses is feasible. We investigate three different approaches for ultrashort pulse generation via THz quantum cascade lasers, namely passive, hybrid and colliding pulse mode locking.