The strong confinement obtained in whispering gallery mode (WGM) resonators is highly conducive to nonlinear effects, due to both resonant enhancement of the fields and a large modal overlap. This confinement can, however, also lead to difficulties; it is difficult to efficiently pump the nonlinear interaction whilst also extracting the signal light, especially when very different wavelengths are involved.
The common coupling mechanisms to WGM resonators are all evanescent, and the coupling rates inherit the exponential decay of the evanescent field. The decay length of these fields is proportional to the wavelength, so longer wavelength modes will tend to couple more than their shorter counterparts. Experimentally this hinders efficiency and, by extension, observation of nonlinear processes.
Through the use of a birefringent prism, and the different phase-matching conditions it imposes on coupling orthogonally-polarized modes, we can independently control the coupling rates of a pump mode and its second harmonic in an x-cut lithium niobate resonator. We thereby critically couple pump and signal in kind, increasing the process’s efficiency fifteen-fold.
This selective coupling can easily be applied to birefringent resonators, where the birefringence is large enough, through use of a prism of the same material. This can also be used with isotropic media, if a suitable birefringent material can be found.
Long photon confinement and high optical fields require good optical resonators. Some of the best optical resonators with a small footprint are whispering gallery mode (WGM) resonators. Their principle is based on continuous total internal reflection at the interface of a round dielectric. Currently most WGM resonators are fabricated fully symmetric to their rotational axis. In WGM resonators fabricated from uniaxial crystals this symmetry axis then coincides with the optic axis, such that the modes are either parallel or perpendicular polarized. If the optic axis is however tilted with respect to the symmetry axis the polarization of the modes changes dramatically. We report on high <i>Q</i> resonances in a slightly birefringent MgF<sub>2</sub> WGM resonator, cut at an angle of 20° with respect to the optic axis. A novel type of mode is observed that can be fully coupled (decoupled) with a right (left) hand circular polarized beam of light. Furthermore, the polarization properties at different outcoupling positions, determined via full Stokes measurements, are recorded and show a continuous complex change in ellipticity. We present the experimental results. Understanding the polarization behavior in an off-axis, birefringent WGM resonator may offer a new way for phase-matching in non-linear χ<sup>(2)</sup> materials.