Exceptional points in coupled microdisk THz quantum cascade lasers

Summary form only given. We present experimental results on coupled pairs of microdisk quantum cascade lasers (QCLs) emitting in the terahertz (THz) spectral region. This resonator type allows to control and to strongly confine the optical mode within the cavity. The coupling is based on the evanescent field of whispering gallery modes (WGMs) formed in each disk. Theoretical calculations predict that counterintuitive effects like pump induced ceasing of lasing due to an exceptional point can be expected in this system. Since QCLs are electrically pumped devices the gain and loss properties of each coupled disk can be individually adjusted electrically, making them ideal candidates for the investigation of these effects. The THz spectral region is chosen for the experiments providing long range evanescent fields which are essential for efficient coupling. A further advantage is that devices with large dimensions can be fabricated which still operate in the single mode emission regime in order to avoid undesired mode competition effects. We realized devices which make use of a double-metal cavity, where the active region is in between 2 metal layers acting simultaneously as electrical contact and waveguide layer. In order to observe the predicted effect additional losses have to be introduced to the lasers. Therefore we reduced the thickness of the waveguide and implemented absorption layers. The disks are fabricated in close vicinity in order to enhance the coupling of the exponentially decaying evanescent field.