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 using two semiconductor layers (such as silicon), one of which includes a heater and the other includes a thermally tunable optical waveguide. Spatially separating these two functions in the optical device results in more efficient heat transfer between the heater and the optical waveguide, reduced heat transfer to the surroundings, and reduced optical losses in the optical waveguide relative to existing silicon-based optical devices.