Ultrahigh-quality-factor silicon-on-insulator microring resonator

The development of ultrahigh-quality-factor $\left(Q\right)$ silicon-on-insulator (SOI) microring resonators based on silicon wire waveguides is presented. An analytical description is derived, illustrating that in addition to low propagation losses the critical coupling condition is essential for optimizing device characteristics. Propagation losses as low as $1.9\pm 0.1 \mathrm{dB}/\mathrm{cm}$ in a curved waveguide with a bending radius of $20 \mathit{\mu }\mathrm{m}$ and a $Q$ factor as high as $139.000\pm 6.000$ are demonstrated. These are believed to be the highest values reported for a curved SOI waveguide device and for any directly structured semiconductor microring fabricated without additional melting-induced surface smoothing.