A Ni based alloy, which consists of by mass percent, C\u22660. The welding wires are composite materials comprising a metal alloy and high temperature nanoparticles dispersed in the metal alloy. The invention is characterized in that it comprises at least a step of epitaxial lateral overgrowth and in that it comprises a step which consists in separating part of the GaN layer from its substrate by embrittlement through direct ion implantation in the GaN substrate. The key switch is disposed between the battery and the ignition circuit and has an ignition terminal. Advantageously, the temperature insensitive optical retarder is simple, easy to construct, can be used for any birefringent crystal and does not require the use of different birefringent crystals.