Unconventional modes induced chiral symmetry breaking in optical microcavity

In the past decade, great efforts have been devoted to manipulate lights in optical microcavity. Particularly, breaking the chiral symmetry is one of the essential targets to achieve due to its broad applications in signal processing, optical sensing, and nonlinear optics. Herein, we theoretically demonstrate the broken symmetry between clockwise (CW) and counter-clockwise (CCW) waves in microcavity coupled with a Fabry-Pe ' rot (F-P) cavity. By introducing loss into part of the F-P cavity, the unconventional modes with strong localization can be realized. In this sense, the modified mode of the external cavity will transport unbalanced light intensity into the CW and CCW waves of the microcavity, sequently breaking the symmetry between these two components. The chirality alpha of 0.925 can be obtained by tuning the loss of the F-P cavity. Leveraging on the generated chirality, unambiguous unidirectional emission is realized when a bus-waveguide is arranged near the microcavity. Furthermore, our numerical results confirm that the unidirectional emission shows ultra-sensitivity to the external perturbations such as nano particles. Nano particles as small as 1-7 nm can be effectively recognized by comparing the light intensity delivered to the two ports of the waveguide. We believe this work is essential for studying the rich physics in non-Hermitian systems and advancing the potentials of microcavity.