Diffractive Structures Within Polymer Substrates, Their Manufacture and Use

A substrate sheet comprising a material having a general refractive index n, the substrate sheet containing at least one ordered two-dimensional array of discrete laser-modified tracks in the material generated by a beam-shaped laser with laser light distributed along and/or about a laser propagation path extending within the substrate sheet,

• each laser-modified track comprising an elongate volume of modified substrate material at least 4 times longer than its narrowest width extending at least partially across a thickness of the substrate sheet, that comprises a modified form of the substrate material, that has a refractive index that is different to the general refractive index n of the substrate sheet from which each laser-modified track originated from, wherein for each two-dimensional ordered array the laser-modified tracks collectively diffract light impinging on the substrate sheet to form an observable shape, image, or region of colour,
• the substrate sheet optionally further comprising one or more additional layers that partially or entirely overlay the substrate material, the one or more additional layers each independently selected from a polymer layer, a reflective layer, a refractive layer, a diffractive filter, a transmissive filter, a protective layer, an overcoat, an adhesion promoter layer, an ink, an optical interference layer and an optical interference stack.