PCT/SG2019/050519 WO 2020/081014 be deposited (in block 604) to form the second waveguide section. [0073] The methods shown in FIGs. 7A-7D are very similar to those shown in FIGs. 6A-6D and described above. As shown in the diagrams, instead of forming the second waveguide section (in block 604) and the resistive heating pad (in block 612) in separate processing stages, the formation of the structures (the second waveguide section 402 and resistive heating pads 404A, 404B) is now linked. Initially, undoped regions, which will ultimately form the resistive heating pads, are formed at the same time as the second waveguide section (block 704) and then those regions are doped (block 705). The undoped regions that are formed from the second material at the same time as the second waveguide section (in block 704), have the dimensions of the resistive heating pads and the doping changes the thermal and electrical properties of the regions. Although not shown in FIGs. 7A-7D, it will be appreciated that the formation of the second waveguide section 402 (in block 704) may involve an etching process, or two separate deposition stages, in order to form the stepped cross-section as shown in FIG. 4E. [0074] The methods shown in FIGs. 8A-8D are very similar to those shown in FIGs. 6A-6D and described above. As shown in the diagrams, instead of forming a single resistive pad on top of a cladding layer, the resistive pads are formed directly on the second waveguide section (in block 812). Although not shown in FIGs. 8A-8D, it will be ap preciated that the formation of the second waveguide section 402 (in block 804) may involve an etching process, or two separate deposition stages, in order to form the stepped cross-section as shown in FIG. 4G. [0075] Depending upon the materials used to form the cladding and waveguide sections, the techniques used to form them may be different. For example, where the second waveguide section is formed from crystalline silicon, this cannot be formed by de position and hence the cladding layer below the second waveguide section (e.g. the first cladding section) may be a buried oxide (BOX) layer. Furthermore, because crystalline silicon cannot be formed by deposition, crystalline silicon cannot be used to form the second waveguide section in the variants of the improved tunable element shown in FIGs. 3C and 4C. If silicon is used to form the second waveguide section in these variants, then poly-crystalline or amorphous silicon may be used. [0076] A first further example provides a tunable element for an optical waveguide device, the tunable element comprising: a first waveguide section formed from a first material; a second waveguide section formed from a second material and arranged to receive light coupled from the first waveguide section; a third waveguide section formed from the first material and arranged to receive light coupled from the second waveguide section; and one or more resistive heating pads proximate to the second waveguide section, wherein the second material is more thermo-optically sensitive than the first