PCT/SG2019/050519 WO 2020/081014 performed in different orders in order to fabricate the different layer structures shown in FIGs. 3B-3D, 3G, 4B-4D and 4G. There may also be additional manufacturing stages which are not shown in FIGs. 6A-6D, 7A-7D and 8A-8D, such as deposition of additional metal layers, cleaning stages, etc. [0070] FIG. 6A shows an example method of fabricating an improved tunable element having a cross-section as shown in FIG. 3B. A first cladding layer is formed (block 602) and this may be formed directly on a substrate (e.g. a silicon wafer) or on a part- processed substrate (e.g. such that the cladding is not formed directly on the surface of the substrate but on materials that have already been deposited on the substrate). In various examples, this first cladding layer may comprise a buried oxide layer. The second waveguide section 302 may then be formed from the second material on top of the first cladding layer (block 604) before a second cladding layer is deposited (block 606). The first and third waveguide sections 301, 303 may then be formed at the same time on top of the second cladding layer (block 608) before a third cladding layer is deposited (block 608). The resistive heating pad 304 is then deposited on top of the third cladding layer (block 612) and then covered by a fourth cladding layer (block 614). [0071] FIG. 6B shows an example method of fabricating an improved tunable element having a cross-section as shown in FIG. 3C. This method is similar to that shown in FIG. 6A and described above (e.g. in that it forms a multi-layer waveguide structure); however, as shown in FIG. 6B, the first and third waveguide sections 301, 303 are formed at the same time on top of the first cladding layer (block 608) before the second cladding layer is deposited (block 606). The second waveguide section 302 is then formed on top of the second cladding layer (block 604) before a third cladding layer is deposited (block 608). As before, the resistive heating pad 304 is then deposited on top of the third cladding layer (block 612) and then covered by a fourth cladding layer (block 614). [0072] FIGs. 6C and 6D show two different example methods of fabricating an improved tunable element having a cross-section as shown in FIG. 3D. The methods of FIGs. 6C and 6D are similar to those shown in FIGs. 6A and 6B; however, as the first, second and third waveguide sections 301-303 are substantially co-planar, there is one fewer cladding deposition step (i.e. block 614 is omitted) and the second waveguide section 302 may be formed (in block 604) either directly after (as in FIG. 6C) or directly before (as in FIG. 6D) the forming of the first and third waveguide sections 301, 303 (in block 608). In the method of FIG. 6C, a continuous waveguide core may be formed initially from the first material (in block 608) such that the first and third waveguide sections are connected and then (also in block 608), a portion of the waveguide formed from the first material may be removed to leave a gap where the second material can