Universal output directionality of single modes in a deformed microcavity

We have investigated the characteristics of quasibound states of a quadrupole-deformed microcavity both experimentally and theoretically. Five distinct single-mode groups are observed in the cavity emission spectra with their cavity quality factors varied by several orders of magnitude. Although the corresponding mode wave functions show qualitatively different geometrical shapes, these single modes exhibit almost identical far-field emission patterns, which contradicts a common understanding that the emission direction is usually associated with the geometry of the mode wave functions. The observed universal directionality is explained by a subtle manifestation of unstable manifolds of classical chaos in the formation of quasibound states.