Chirality of exceptional points in bianisotropic metasurfaces

In optical systems, one kind of exceptional point (EP) is associated with the maximally unidirectional reflection. Here, we theoretically show that the intrinsic chirality of this kind of EP is only determined by the sign of the scattering rate difference, and that these EPs could only be on or within a fundamental scattering bound in an asymmetric resonant system. As a proof of our theoretical deviation, a bianisotropic metasurface is designed to exhibit an extreme EP with a definite chirality on the fundamental scattering bound. In addition, another EP with the opposite chirality is also available within this scattering bound in the same metasurface without any additional symmetry operation. Numerical results are in good agreement with our theoretical predictions based on the coupled mode theory. We believe that our results not only provide physical insights to explore EPs in resonant systems, but also have implications in designing unidirectional absorbers and thermal emitters.