New Modes in Old Cavities: The Surprising Effects of Boundary Phase Shifts

Multilayer reflective boundaries used to guide and resonate light impart reflective phase shifts that depend on both angle of incidence and polarization. In cavities, these phase shifts can have a large effect on the spatial and polarization patterns of the eigenmodes, creating completely new structures. Here I discuss two new families of modes found in simple dome-shaped cavities. Both families owe their existence to derivatives of the boundary phase shift with respect to incident angle. The Goos-Haenchen (GH) modes are the first known cavity modes to be created by the well known GH shift, and are beautifully explained in terms of a saddle-node bifurcation in the phase space of the GH-modified ray dynamics. The second family, the paraxial mixed modes, possess mixtures of different spin and orbital angular momentum despite their trajectory along the axis of a cylindrically symmetric system. (Total angular momentum remains a good quantum number as enforced by symmetry.) The mixed modes are the answer to the question "What happens to the eigenmodes when the cavity linewidth is small enough to resolve the breaking of the approximate, textbook degeneracy of the paraxial Gaussian modes of transverse order N?"