Embodiments of an optical device, an array of optical devices, and a technique for fabricating the optical device or the array are described. This optical device is implemented on a substrate (such as silicon), and includes a thermally tunable optical waveguide that has good thermal isolation from its surroundings. In particular, a portion of a semiconductor in the optical device, which includes the optical waveguide, is free standing above a gap between the semiconductor layer and the substrate. By reducing the thermal coupling between the optical waveguide and the external environment, the optical device can be thermally tuned with significantly less power consumption.