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

TLDR

Atomic‑force‑microscopy data combined with a simple surface‑scattering model indicate that residual surface inhomogeneities limit the quality factor of fused‑silica microspheres. The authors measured a quality factor of ~8 × 10⁹ (finesse ~2.2 × 10⁶) for whispering‑gallery modes at 670–850 nm, found Q to be wavelength‑independent, and attributed the remaining losses to surface scattering and absorbed water rather than bulk absorption.

Abstract

Measurements of the quality factor Q?8×109 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 F?2.2×106. ?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.