Diffractive structures within polymer substrates, their manufacture and use

A method for the manufacture of a security document or security device, the method comprising the steps of:

• providing a substrate sheet comprising a substrate sheet material;
• irradiating the substrate sheet at a plurality of discrete positions across a planar side of the substrate sheet corresponding to a two-dimensional array, with a laser beam from a femtolaser, whilst modifying the shape of the laser beam prior to its interaction with the substrate sheet so that the incident laser light is at least partially distributed along and / or about a laser propagation path extending within the substrate sheet;
• to at least partially or temporarily melt, displace or decompose at least a portion of the substrate material at or within an elongate volume of the substrate sheet material about the laser beam longitudinal axis, corresponding to each of said discrete positions,
• thereby to generate an array of laser-modified tracks within the substrate material,
• each laser-modified track comprising an elongate volume of modified substrate material at least 4 times longer than its width extending at least partially across a thickness of the substrate sheet, where the modified form of the substrate material has a refractive index that is different to the general refractive index n of the unmodified substrate;
• wherein each two-dimensional ordered array of laser-modified tracks thus produced collectively diffract light impinging on the substrate sheet to form an observable shape, image, or region of colour.