Rayleigh scattering, mode coupling, and optical loss in silicon microdisks

High refractive index contrast optical microdisk resonators fabricated from silicon-on-insulator wafers are studied using an external silica fiber taper waveguide as a wafer-scale optical probe. Measurements performed in the $1500\mathrm{nm}$ wavelength band show that these silicon microdisks can support whispering-gallery modes with quality factors as high as $5.2\times {10}^5$ , limited by Rayleigh scattering from fabrication induced surface roughness. Microdisks with radii as small as $2.5\mu \mathrm{m}$ are studied, with measured quality factors as high as $4.7\times {10}^5$ for an optical mode volume of $5.3{(\lambda ∕n)}^3$ .