A method of trimming the refractive index of material forming at least part of one or more structures integrated in one or more pre-fabricated devices, the method comprising: implanting one or more first regions of material of one or more pre-fabricated devices, encompassing at least partially one or more device structures, with ions to alter the crystal form of the material within the one or more first regions and change the refractive index of the material within the one or more first regions; and heat treating one or more second regions of material of the one or more devices, encompassing at least partially the one or more first regions, to alter the crystal form of the material within the one or more first regions encompassed by the one or more second regions and change the refractive index thereof, thereby trimming the refractive index of the material of at least part of the one or more device structures, such that the one or more device structures provide one or more predetermined device outputs.