Tailoring the behavior of optical microcavities with high refractive index sol-gel coatings

Silica optical resonators with high quality factors, small mode volumes, and high refractive index contrast are desirable in many applications throughout the fundamental and applied sciences. However, these parameters are not easily modified in plain silica devices. In the present work, we theoretically and experimentally demonstrate the use of high refractive index sol-gel coatings to achieve a significant reduction in mode volume of silica toroids while maintaining high quality factors. In addition to tuning the mode volume, the presence of the coating changes both the location and shape of the optical field.