Divergent Petermann factor of interacting resonances in a stadium-shaped microcavity

It is numerically shown that the Petermann excess noise factor diverges to infinity at an exceptional point (EP) found near an avoided resonance crossing in a stadium-shaped dielectric microcavity. Square-root singularity of eigenvalue at the EP is explicitly demonstrated in two-dimensional parameter space spanned by refractive index and deformation parameter. The results imply that mode-mode interaction makes the Petermann factor large.