Marginally unstable periodic orbits in semiclassical mushroom billiards

Optical mushroom-shaped billiards offer a unique opportunity to isolate and study semiclassical modes concentrated on nondispersive, marginally unstable periodic orbits. Here we show that the openness of the cavity to external electromagnetic fields leads to unanticipated consequences for the far-field radiation pattern, including directional emission. This is mediated by interactions of marginally unstable periodic orbits with chaotic modes. We also show that the semiclassical modes are robust against perturbations to the shape of the cavity, despite the lack of structural stability of the corresponding classical orbits.