Photonic molecule based on coupled ring quantum cascade lasers
Photonic molecules - particular systems composed of coupled optical resonators - emulate the behavior of complex physical systems exhibiting discrete energy levels. In this work, we present a novel photonic molecule composed of two strongly coupled, mid-infrared ring quantum cascade lasers. We explore both experimentally and numerically the key features of the photonic molecule such as the energy level splitting of bonding and antibonding supermodes. Due to the large size of the resonators, the energy splitting results in bands containing tens of modes. Each of these modes is furthermore doubly degenerate with respect to the direction of propagation, namely clockwise and counterclockwise. We explore several methods to carefully break these symmetries of the system in a controlled manner by introducing spatial and temporal asymmetries in the pumping scheme of the ring lasers. By employing these techniques, we achieve a high degree of precision in the dynamic control of the photonic molecule. Owing to their inherent suitability for on-chip integration, this new class of devices may enable applications as varied as novel mid-infrared sensors or a rich playground for studying non-Hermitian photonics and quantum optics with quantum cascade lasers.