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 1500nm wavelength band show that these silicon microdisks can support whispering-gallery modes with quality factors as high as 5.2×105, limited by Rayleigh scattering from fabrication induced surface roughness. Microdisks with radii as small as 2.5μm are studied, with measured quality factors as high as 4.7×105 for an optical mode volume of 5.3(λ∕n)3.