Mechanism of Q-spoiling in deformed optical microcavities.

It was reported that Q spoiling in a chaotic microcavity is caused by chaos [PRL, 75, 2682 (1995)] and chaos-assisted tunneling [Nature 385, 45 (1997)]. However, even when a cavity is slightly deformed not to exhibit a broad chaotic region in phase space, high Q modes are spoiled. We find that Q spoiling in this region is caused by the transition of a whispering gallery mode (WGM) to a scarred resonance when a WGM interacts with its pair quasi-normal mode through an avoided resonance crossing. We prove that this transition induces Q spoiling in a quadrupole dielectric microcavity by showing that Q factors obtained from the Husimi functions depending on resonance deformation during the transition agree well with those obtained from the complex eigenvalues.