  Non-Hermitian exceptional points (EPs) represent a special type of degeneracy
where not only the eigenvalues coalesce, but also the eigenstates tend to
collapse on each other. Recent studies have shown that in the presence of an
EP, light-matter interactions are profoundly modified, leading to a host of
novel optical phenomena ranging from enhanced sensitivity to chiral light
transport. As of now, however, in order to stabilize a system at the vicinity
of an exceptional point, its related parameters must be carefully tuned and/or
continuously controlled. To overcome this limitation, here we introduce a new
family of broadband exceptional points based on unidirectional coupling,
implemented by incorporating an Sshaped waveguide in a microring cavity. In
active settings, the resulting unidirectionality exhibits unprecedented
resilience to perturbations, thus providing a robust and tunable approach for
directly generating beams with distinct orbital angular momentum (OAM). This
work could open up new possibilities for manipulating OAM degrees of freedom in
applications pertaining to telecommunications and quantum computing, while at
the same time may expand the notions of non-Hermiticity in the orbital angular
momentum space.
