  A laser exhibits both controllable gain and loss and, under proper design
conditions, is an ideal non-Hermitian system allowing the direct observation
and engineering of spectral singularities such as exceptional points (EPs). A
dual section distributed feedback (DFB) quantum cascade laser (QCL) is a
prototype of such a system, allowing the controlled coupling of a ladder of
cavity Fabry-Perot (FP) modes to a quarter wave shifted DFB mode. Tuning the
coupling strength and the gain difference between these two set of modes
enables probing the regimes from weak coupling to strong coupling and the
robust observation of exceptional point singularities. At these exceptional
points, the laser exhibits a sequence of lasing and coherent prefect absorption
dynamics1,2 when pumped above transparency. Additionally, the pumping scheme
allows the deliberate lifting of the exceptional point degeneracy. These
results show that dual section QCL is a perfect platform to study exceptional
points because the coupling parameter and system loss can be tuned in a single
device.
