  We report a surprising observation that the output directionality from
wavelength-scale optical microcavities displays extreme sensitivity to
deformations of the cavity shape. A variation of the cavity boundary on the
order of ten thousandth of a wavelength may flip the output directions by 180
degrees. Our analysis based on a perturbation theory reveals that a tiny shape
variation can cause a strong mixing of nearly degenerate cavity resonances with
different angular momenta, and their interference determines the farfield
emission pattern. This work shows the possibility of utilizing
carefully-designed wavelength-scale microcavities for high-resolution detection
and sensing applications.
