  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 nm
wavelength band show that these silicon microdisks can support
whispering-gallery modes with quality factors as high as 5.2 x 10^5, limited by
Rayleigh scattering from fabrication induced surface roughness. Microdisks with
radii as small as 2.5 microns are studied, with measured quality factors as
high as 4.7 x 10^5 for an optical mode volume of 5.3 cubic wavelengths in the
material.
