  Recently, an interesting phenomenon of spatial localization of optical modes
along periodic ray trajectories near avoided resonance crossings has been
observed [J. Wiersig, Phys. Rev. Lett. 97, 253901 (2006)]. For the case of a
microdisk cavity with elliptical cross section we use the Husimi function to
analyse this localization in phase space. Moreover, we present a semiclassical
explanation of this phenomenon in terms of the Goos-Haenchen shift which works
very well even deep in the wave regime. This semiclassical correction to the
ray dynamics modifies the phase space structure such that modes can localize
either on stable islands or along unstable periodic ray trajectories.
