Local chirality of optical resonances in ultrasmall resonators.

In wavelength-scale cavities with chiral-symmetric geometry, wave optical effects can introduce local chirality, that is, a spatial separation of the clockwise and counterclockwise propagating resonant modes. We show that this local chirality results in unidirectional lasing emission in the far field. In the presence of a waveguide, the local chirality also allows for directional evanescent coupling of the lasing modes, and the output direction can be varied by selecting the coupling position along the cavity boundary. Our results demonstrate that the local chirality of optical resonances can be utilized to control the output directionality and enhance the collection efficiency of emission from ultrasmall resonators.