Holographic watermarks and steganographic markings for combating the counterfeiting practices of high-value metal products

This paper describes recent advances in laser direct writing of tamper-proof holographic structures on metal surfaces for preventing counterfeiting of high-value metal products, e.g. luxury watches, medical tools and implants, collectible coins, etc. Each of these holographic structures consists of an array of optically-smooth craters arranged in such a way to generate diffractive images comprising, e.g. a company logo and/or a string of alphanumeric characters, providing a unique method for the traceability of genuine products. The craters are less than 10 μm across and less than 500 nm deep. They are generated on metals by UV nanosecond laser pulses (355 nm wavelength and 35 ns pulse duration) that lead to localized melting and evaporation of the material. This paper demonstrates various methods for combining the holographic structures with standard marking patterns, such as QR codes and Data Matrices, in order to form aesthetic holographic markings concealing secret messages about the products. By merging a few holographic patterns together it is also possible to generate so called “holographic watermarks”. Finally, this article describes a few approaches for making the holographic structures particularly difficult to replicate and counterfeit. This includes the generation of multi-level holograms as well as the formation of optically-smooth protrusions (bumps) in selected places within the holographic structures in order to create hidden identifiers and/or miniature signatures which cannot be detected by the naked eye.