Beyond the Rayleigh scattering limit in high-Q silicon microdisks:: theory and experiment

Using a combination of resist reflow to form a highly circular etch mask pattern and a low-damage plasma dry etch, high-quality-factor silicon optical microdisk resonators are fabricated out of silicon-on-insulator (SOI) wafers. Quality factors as high as Q = 5 x 10(6) are measured in these microresonators, corresponding to a propagation loss coefficient as small as alpha similar to 0.1 dB/cm. The different optical loss mechanisms are identified through a study of the total optical loss, mode coupling, and thermally-induced optical bistability as a function of microdisk radius (5-30 mum). These measurements indicate that optical loss in these high-Q microresonators is limited not by surface roughness, but rather by surface state absorption and bulk free-carrier absorption. (C) 2005 Optical Society of America.