Unidirectionally coupled microcavities: intuitive and robust construction of high-order exceptional points

In recent years, non-Hermitian degeneracies, so-called exceptional points, have attracted immense attention in photonics and optics. Among a range of interesting fundamental aspects, exceptional points have great potential for applications, such as topological mode switching, non-reciprocal devices, and ultrasensitive sensors. Although the research on exceptional points has grown enormously in the last years, the construction of high-order exceptional points, where more than two eigenfrequencies and corresponding modes coalesce, is still challenging. <br/> <br/> We tackle this challenge by a robust implementation of high-order exceptional points in microring cavities. To do so, we combine the known mirror-induced asymmetric backscattering with the unidirectional coupling between microcavities via adjacent waveguides. We investigate the topology of eigenfrequencies around the exceptional points and demonstrate a cavity-selective sensing scheme. Our numerical results are also intuitively described by effective Hamiltonians. Our scheme allows for the robust and scalable construction of high-order exceptional points in integrated semiconductor platforms.