PCT/SG2019/050519 WO 2020/081014 [0039] The first and third sections may be referred to as ‘input’, ’output’ or ‘input/output’ sections of waveguide and the second section may be referred to as the ‘centre’ section of waveguide. As described above, light propagates through the first section, then through the second section and then through the third section (or in the opposite order, for light propagating in the opposite direction). [0040] The term ‘thermo-optic sensitivity’ is used herein to refer to the change in refractive index with temperature (denoted dn/dT, where dn is the change in refractive index that occurs for a change in temperature dT), with a higher thermo-optic sensitivity referring to a material with a larger value of dn/dT. [0041] By using a material with a higher thermo-optic sensitivity for only the portion of the waveguide that is proximate to the resistive heating pad, i.e. the second section, any phase change that is caused by unintentional heating of the other (less thermo-optically sensitive) waveguide material, i.e. the first and/or third section, will be significantly smaller than the phase change caused by heating of the second section. This phase change due to unintentional heating (e.g. heat spreading beyond the second section of waveguide) contributes to the phase error and hence the phase error is reduced compared to a tuning element (e.g. as shown in FIGs. 2A-2C) where the entire waveguide (i.e. waveguide core) is formed from the same material. [0042] Additionally, by using a material with a higher thermo-optic sensitivity for the portion of the waveguide that is proximate to the resistive heating pad, i.e. the second section, less energy is required to produce a desired phase change in the propagating light and hence the overall tunable element 300 is more efficient compared to a tunable device with a waveguide formed entirely from the first material. [0043] Additionally, by only using the material with higher thermo-optic sensitivity for the portion of the waveguide that is proximate to the resistive heating pad, i.e. the second section, and not for the rest of the waveguide, i.e. the first and third sections, the overall optical loss of the system may be reduced. For example, where Si is used for the second section, the waveguide loss may be l.5dB/cm, whereas the waveguide loss for SiN (which may be used for the first and third sections) may be less than 0.5dB/cm. [0044] FIG. 3A shows a schematic diagram (in planar view) of an example improved tunable element 300. This improved tunable element may, for example, be in corporated in a waveguide device such as shown in FIGs. 1A and 1B (e.g. tunable element 112). The improved tunable element 300 comprises three waveguide (i.e. waveguide core) sections 301-303 and a resistive heating pad 304 (which may, for example, be formed from titanium nitride, TiN). The direction of propagation of the light is shown by the arrows in FIG. 3A although as described above (e.g. with reference to FIG. 1B), in various examples the light may also propagate in the opposite direction through the tunable element 300.