Abstract: An asymmetric thermo-optical switch uses materials having different dn/dT coefficients. A first waveguide is constructed on a planar substrate using a material that has a first dn/dT coefficient. A second waveguide is constructed on the planar substrate using a second material that has a second dn/dT coefficient. In a first embodiment, a coupling region is formed between the first and second waveguides. An index-adjusting switch element is deposited on a first side of the device over the coupling region to control the proportion of light coupled between the waveguides. In a second embodiment, a splitting region is formed by connecting the second waveguide to the first waveguide, at a middle portion of the first waveguide. An index-adjusting switch element is deposited on a first side of the device over the splitting region to control the proportion of light split between the waveguides.