Ultimate quality factor of silica microtoroid resonant cavities

Silica optical microcavities with quality (Q factors above $1×108$ have applications throughout science and engineering. While both the microtoroid and microsphere resonant cavity have demonstrated $Q>1×108$⁠, only the microsphere has surpassed $1×109$⁠. Surprisingly, the reason for this performance disparity is directly related to type of silicon substrate used in the fabrication process. In the present work, the theoretical Q of planar toroidal silica resonant cavities is calculated and compared to experimental results from a series of devices fabricated from oxide on doped silicon wafers. As predicted, the Q depends on the substrate dopant concentration.