Abstract: A method of forming a film of photonic crystal material. A first process is performed upon a material capable of having a photonic crystal structure, this process causing deformation of the material so as to form a film in which incident light received by the material is selectively reflected or transmitted to generate a first optical effect in the film. A second process is performed upon substantially all of the film which applies a shear stress to the film. This causes a change in the material structure so as to generate a second optical effect in the film, different from the first optical effect, in response to incident light. Security films, devices, articles and documents formed using the method are also discussed.

Abstract: A polymer opal material comprises a three dimensionally periodic arrangement of core particles in a matrix material and exhibits structural color via Bragg reflection. IN a process for manufacturing such a material, a sandwich structure is provided, of a precursor composite material held between first and second sandwiching layers. A relative shear strain of at least 10% is imposed on the precursor composite material by curling the sandwich structure around a roller. The shear strain is cycled, in order to promote the formation of the three dimensional periodic arrangement.

Abstract: A composite optical device has a layer of a composite optical material mounted on a substrate. The layer of composite optical material has substantially uniform thickness. The composite optical material is a polymer opal, in that it has a three dimensional arrangement of core particles distributed in a matrix, the refractive index of the material of the core particles being different to the refractive index of the material of the matrix and the three dimensional arrangement being capable of having a periodicity such that, when a surface of the material is illuminated with white light, the composite material exhibits structural colour. The local stiffness of the substrate is different at different positions of the substrate.

Abstract: A polymer opal material comprises a three dimensionally periodic arrangement of core particles in a matrix material and exhibits structural colour via Bragg reflection. IN a process for manufacturing such a material, a sandwich structure is provided, of a precursor composite material held between first and second sandwiching layers. A relative shear strain of at least 10% is imposed on the precursor composite material by curling the sandwich structure around a roller. The shear strain is cycled, in order to promote the formation of the three dimensional periodic arrangement.

Abstract: A method of forming a film of photonic crystal material. A first process is performed upon a material capable of having a photonic crystal structure, this process causing deformation of the material so as to form a film in which incident light received by the material is selectively reflected or transmitted to generate a first optical effect in the film. A second process is performed upon substantially all of the film which applies a shear stress to the film. This causes a change in the material structure so as to generate a second optical effect in the film, different from the first optical effect, in response to incident light. Security films, devices, articles and documents formed using the method are also discussed.