Local chirality of optical waves in ultrasmall resonators

The local chiral symmetry between clockwise (CW) and counter-clockwise (CCW) propagating light in a deformed microcavity can be broken by wave optics effects, which become significant as the cavity size approaches the wavelength. We show that the spatial separation of the CW and CCW ray orbits underlying the high quality factor resonant modes results in unidirectional emission in free space. In the presence of a waveguide, evanescent coupling also becomes directional, and the output direction can be varied by selecting the coupling position along the cavity boundary. Our results demonstrate that the local chirality can be utilized to control the output directionality and enhance the collection efficiency of emission from ultrasmall resonators.