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 with a high thermal resistance to the surrounding external environment and a low thermal resistance to a localized heater. In particular, the thermal resistances associated with thermal dissipation paths from a heater in the optical device to an external environment via electrodes and via the substrate are increased, while the thermal resistance between the optical waveguide and the heater is decreased.