Unidirectional reflection from an integrated“taiji”microresonator

We study light transmission and reflection from an integrated microresonator device,formed by a circular microresonator coupled to a bus waveguide,with an embedded S-shaped additional crossover waveguide element that selectively couples counter-propagating modes in a propagation-direction-dependent way.The overall shape of the device resembles a“taiji”symbol,hence its name.While Lorentz reciprocity is preserved in transmission,the peculiar geometry allows us to exploit the non-Hermitian nature of the system to obtain high-contrast unidirectional reflection with negligible reflection for light incident in one direction and a significant reflection in the opposite direction.