Birefringent crystals like lithium niobate show a very strong χ (2) non-linear response and have thus been used in a whispering galley mode (WGM) supporting geometry to achieve extremely high conversion efficiencies [1,2]. In a uniaxial crystal, light is split into two different eigenpolarizations showing different phase velocities. Usually the WGM resonators are fabricated such that the optic axis is parallel to the symmetry axis, such that there are still two mode families of parallel (perpendicular) polarization with respect to the optic axis, namely TE (TM) modes, which "feel" the extraordinary (ordinary) index of refraction of the material respectively. The dispersion between these two types of modes can be exploited to achieve phasematching in nonlinear processes.We report on a novel geometry: A WGM resonator made of single crystal magnesium fluoride whose optic axis is tilted against its symmetry axis by a large angle Ψ = 20° (see Fig. 1b). Similarly to the polarization effects of a birefingent wave plate we expect some new features in the polarization behaviour of the resonator modes. In the experimental setup shown in Fig. 1a we can couple fully polarization classified light into and out of the resonator at variable positions with two prism couplers. We perform a full Stokes measurement of the out-coupled light at different positions along the resonator. The observed resonance spectrum is very similar to that of an "usual" WGMR. There are modes with reasonable coupling efficiency and high qualtiy factor. The polarization of these modes however is quite unusual. We observe modes which show best coupling when using fully circular polarized light (see Fig. 1b). At different out-coupling locations the Stokes measurements show a rather complex elliptical polarization (Fig 1d). Even modes with linear in-coupled light emit, in general, elliptically polarized light.The understanding of this polarization behaviour inside such resonators may open new and interesting applications in the field of nonlinear optics [3] as it could give access to a new type of phase matching in such a system. Also cavity-enhanced biosensing could benefit from such a resonator by using the elliptically polarized evanescent field leaking out of the cavity to measure chirality of molecules.[1] V. S. Ilchenko and L. Maleki, "Novel whispering-gallery resonators for lasers, modulators, and sensors," Proc. SPIE 4270, 120-130 (2001) [2] J.
