Achieving unidirectional emission and manipulating waves in a microcavity are crucial for information processing and data transmission in next-generation photonic circuits (PCs). Here we show how to impose twin microcavities with opposite chirality by incorporating parity-time ($\mathcal{PT}$) symmetry to realize unidirectional emission. Our numerical calculation results show that the opposite chirality in microcavities stems from the asymmetric coupling of the clockwise (CW) and counterclockwise (CCW) components carried by the attached waveguide to the left- or right-sided microcavities, respectively. Notably, by engineering $\mathcal{PT}$ symmetry in the coupled system via the gain-loss control, the clockwise component of the lossy cavity could be selectively suppressed, which leads to the unidirectional emission with an extinction ratio of up to −52 dB. Furthermore, the chirality and $\mathcal{PT}$-symmetry breaking enabled unidirectional emission is extremely sensitive to external scatters, allowing the detection of nanoparticles with an ultrasmall radius of 5–50 nm by recording the extinction ratio change. The proposed system provides a simple yet general way to manipulate the standing waves in a microcavity and will be essential for advancing the potentials of the microcavity in PCs.

• Received 17 July 2019

DOI:https://doi.org/10.1103/PhysRevA.101.013833