  Engineering the transport of radiation and its interaction with matter using
non-Hermiticity, particularly through spectral degeneracies known as
exceptional points(EPs), is an emerging field that has both fundamental and
practical implications. Chiral behavior in the vicinity of EPs opens new
opportunities in radiation control, such as unidirectional reflection or lasing
with potential applications in areas ranging from cavity quantum
electrodynamics and spectral filtering to sensing and thermal imaging. However,
tuning and stabilizing a system to a discrete EP in parameter space is a
challenging task: either the system is operated close to an EP rather than
directly at the EP or the true power of EP is obscured by stability issues.
Here, we circumvent this challenge by designing a photonic system that operates
on a surface of exceptional points, known as an exceptional surface (ES). We
achieve this by using a waveguide-coupled optical resonator with an external
feedback loop that induces a nonreciprocal coupling between its frequency
degenerate clockwise (CW) and counterclockwise (CCW) traveling modes. By
operating the system at critical coupling on the ES, we demonstrate, for the
first time, the effect of chiral and degenerate coherent perfect absorption
(CPA) where input waves in only one direction are perfectly absorbed. This
novel type of CPA-EP is revealed through the observation of the predicted and
long-sought squared Lorentzian absorption lineshapes. We expect our results and
approach to serve as a bridge between non-Hermitian physics and other fields
that rely on radiation engineering.
