424B may be formed from any suitable material, for example, TiN, and may be the same thickness as, or thicker or thinner than the resistive heating pad 304 described above with reference to FIGs. 3A-3G. Whilst FIGs. 4B-4C show the dimensions of the thicker part of the second waveguide section 402A and the first and third waveguide sections 301, 303 (e.g. width and thickness, with the width being measured per pendicular to the direction of propagation and in the plane of the waveguide section and thickness being measured perpendicular to both the direction of propagation and the plane of the waveguide section) being substantially the same, in most examples, the width and/or thickness of the thicker portion 402A of the second waveguide section is different to the width and/or thickness of the first and third waveguide sections 301, 303, with the first and third waveguide sections 301, 303 having identical width and thickness. Whilst the actual dimensions of the different waveguide sections may be chosen when designing the tuning element 400 (or the waveguide device comprising the tuning element 400), if the refractive index of the second material (that forms the second waveguide section 402) is higher than the refractive index of the first material, then propagating light is more strongly confined within the waveguide and the width and/or thickness of the thicker portion 402A of the second waveguide section may be smaller than the width and/or thickness of the first and third waveguide sections 301, 303. In an example, the first and third waveguide sections 301, 303 may be formed from SiN (n=2) and be 400nm thick and lOOOnm wide, whereas the second waveguide section 402 may be formed from Si (n=3.5) and the thicker portion 402A may be 220nm thick and 500nm wide, whilst the outer portions 402B, 402C and resistive heating pads 404A, 404B may be significantly thinner (e.g. a few tens of nanometers). 

[0065] In all the examples described above, the first and third waveguide sections 301, 303 are formed from a first material having a first value of dn/dT, Sl, and the second (or centre) waveguide section 302 is formed from a second material having a second value of dn/dT, S2. The second material is more thermo-optically sensitive than the first material, i.e. S2>Sl. In a variation of any of the examples described above, the first and third waveguide sections may be formed from different materials, e.g. the first waveguide section may be formed from a first material having a first value of dn/dT, Sl, and the third waveguide section may be formed from a third material having a third value of dn/dT, S3, where the second material (that forms the second waveguide section) is more thermo-optically sensitive than both the first material and the third material, i.e. S2>Sl and S2>S3, 

[0066] The improved tunable elements 300, 320, 400, 420 described above with reference to FIGs. 3A-3G and 4A-4G may be incorporated into (i.e. form part of) any tunable waveguide device and two examples, 100, 120 are shown in FIGs. 1A and 1B. Another example tunable waveguide device 500 is shown in FIG. 5. The tunable waveguide 

