Thermally tunable silicon racetrack resonators with ultralow tuning power

We present thermally tunable silicon racetrack resonators with an ultralow tuning power of 2.4 mW per free spectral range. The use of free-standing silicon racetrack resonators with undercut structures significantly enhances the tuning efficiency, with one order of magnitude improvement of that for previously demonstrated thermo-optic devices without undercuts. The 10%-90% switching time is demonstrated to be ~170 µs. Such low-power tunable micro-resonators are particularly useful as multiplexing devices and wavelength-tunable silicon microcavity modulators.