Laser processing of nano-foils in confined environment for security coding

Strips of thin (20 nm thick) aluminium foils laminated on both sides with transparent polymers are processed with a Q-switched Nd:YAG laser to generate specific microscopic features for security coding. The effect of laser processing parameters on material behaviour in the confined environment is analysed. Because of the confinement by the polymers, the particles generated (either by laser melting or vaporisation) of 100 nm to 2 mm in size are trapped and distributed within the polymer films not normally seen in open surface laser beam processing. This produces various microscopic features that have distinct "signatures"dentification. The interaction of the laser beam with the Al foil with a thickness close to the optical absorption length of the beam is another aspect that is different from laser processing of thicker materials. The thermal heating and cooling effects for laser processing of ultra-thin metallic foils together with the mechanisms of various microscopic feature formations are discussed.