High-Q measurements of fused-silica microspheres in the near infrared

Measurements of the quality factor $Q\approx 8\times {10}^9$ are reported for the whispering-gallery modes (WGM's) of quartz microspheres for the wavelengths 670, 780, and 850 nm; these results correspond to finesse $\mathcal{F}\approx 2.2\times {10}^6$. The observed independence of $Q$ from wavelength indicates that losses for the WGM's are dominated by a mechanism other than bulk absorption in fused silica in the near infrared. Data obtained by atomic force microscopy combined with a simple model for surface scattering suggest that $Q$ can be limited by residual surface inhomogeneities. Absorption by absorbed water can also explain why the material limit is not reached at longer wavelengths in the near infrared.