WO 2020/081014 PCT/SG2019/050519 waveguide section comprises a centre portion (402A) and two outer portions (402B, 402C), wherein the centre portion is thicker than the outer portions. [Claim 11] The tunable element according to claim 1, wherein the first, second and third waveguide sections (301-303) are formed in a single layer. [Claim 12] The tunable element according to claim 11, wherein in use light is butt- coupled between an end of the first waveguide section and a proximate end of the second waveguide section and is further butt-coupled between an opposite end of the second waveguide section and a proximate end of the third waveguide section. [Claim 13] The tunable element according to claim 11 or 12, wherein the resistive heating pad is formed in a second layer. [Claim 14] The tunable element according to claim 13, further comprising a layer of cladding material between the first layer and the second layer. [Claim 15] The tunable element according to any of claims 1-5, 11 and 12, wherein the second waveguide section comprises a centre portion (402A) and two outer portions (402B, 402C), wherein the centre portion is thicker than the outer portions and wherein the a resistive heating pad is formed on top of each of two outer portions of the second waveguide section. [Claim 16] The tunable element according to any of the preceding claims, wherein the first material is silicon nitride and the second material is silicon, amorphous silicon or polycrystalline silicon. [Claim 17] The tunable element according to any of the preceding claims, wherein each resistive heating pad (304) has a length that is identical to a length of the second waveguide section (302), such that each resistive heating pad is proximate to the second waveguide section and not to the first or third waveguide sections (301, 303). [Claim 18] A tunable optical waveguide device comprising one or more tunable elements according to any of the preceding claims. [Claim 19] An optical phased array comprising one or more tunable elements according to any of claims 1-17. [Claim 20] A method of fabricating a tunable element for an optical waveguide device, the method comprising: forming a first waveguide section and a third waveguide section from a first material (608); forming a second waveguide section from a second material (604, 704); forming at least one layer of cladding material on the second