Controlling the mode volume in high-Q microcavities with high refractive index coatings

Developing optical resonators with high quality factors, small mode volumes, and high refractive index contrast is important for many integrated optics and communications applications. High quality factors and small mode volumes are especially desirable to maximize the circulating intensity and Purcell factor for laser applications. However, controlling an optical resonator's mode volume and refractive index contrast can be difficult as they depend primarily on the inherent material properties of the resonator. One approach to reduce mode volume and control refractive index contrast is to apply high refractive index polymer coatings. However, polymer coatings are not compatible with all fabrication processes and not as robust as silica and silicon. Recently, we developed and characterized high refractive index silica films containing small amounts of titanium dopant. The silica films are fabricated using a sol gel method with methyl triethoxysilane (MTES) and tetraethyl orthosilicate (TEOS) precursors. Depending on the amount of titanium added, the refractive index of the resulting film can be tuned from 1.44 to 1.62, as measured by spectroscopic ellipsometry. By spin coating these high index silica films onto silica toroid resonators, we experimentally and numerically obtain a significant reduction in mode volume while maintaining high quality factors. Additionally, the presence of the high refractive index coating allows tuning of the circulating light's position, shape, and interaction with the coating. Therefore, these high refractive index films offer a useful and more robust method to optimize the properties of optical devices for communications and integrated optics applications.