Method of applying a pattern, and security device for an article

A security device for an article, the device comprising:

• a layered structure comprising a layer of phase change material, the phase change material being thermally switchable between a plurality of stable states having different refractive indices relative to each other, wherein:
• the layer of phase change material comprises a pattern of different refractive indices at least partly defined by a selected portion of the phase change material in the layer being in one of the stable states and a remaining portion of the phase change material being in one or more other stable states;
• and
• the layered structure comprises a pattern of indentations in a surface of the layered structure, the pattern of indentations being spatially registered with the pattern of different refractive indices in the layer of phase change material, wherein the indentations provide a retroreflective behaviour by which incident light is reflected back towards a light source to a greater extent than would have been the case had the surface of the layered structure been simply planar.