Diffractive and coloured films by laser interferometry patterning

Coloured coatings having both selective optical and diffractive properties have been produced by phase-mask laser interference with a UV laser beam (193 nm and 20 ns pulse duration). The method has been demonstrated on Ag films prepared by magnetron sputtering having equivalent thickness ranging from nearly isolated nanoparticles to continuous film. The laser induces periodic patterns formed by non-transformed film areas surrounded by areas containing nanoparticles whose dimensions are controlled through the as-grown film thickness. The nanoparticles are found responsible for a reflectivity band peaking in the range 446–545 nm depending on the thickness and associated to their surface plasmon resonance that provides the “colour”. The morphology of the pattern can be controlled through the laser energy, number of pulses and the phase-mask configuration and has a unique diffraction pattern associated. This approach allows producing customised coloured coatings with an identification feature. Although the concept is shown for a silver film and rectangular patterns, it can easily be extended to many other types of materials or patterns.