Focused ion beam engineered disc resonators

We report the demonstration of whispering gallery mode (WGM) resonators augmented by focused ion beam (FIB) microfabrication. We demonstrate that we can precisely mill features directly into the perimeter of crystalline disc optical resonators. By cutting a narrow opening completely through the disc, we can create full access to the internal modes. Applying FIB techniques, we can also precisely engrave a grating structure on the disc surface. The diffraction grating provides a simple and highly directional free-space coupling mechanism with superior stability to evanescent coupling techniques. These embedded gratings can also provide control of the resonance spectrum, significantly reducing the mode density. This FIB fabrication process does not introduce significant loss; Q ≃ 10⁸ has been demonstrated. The wavelength dependence of the diffraction angle was found to be in excellent agreement with grating theory. The versatility of mode accessibility, spectral control and far-field grating coupling will have significant impact in WGM resonator applications in lasers, sensors, and optoelectronics.