Maximizing the efficiency of laser-fabricated diffraction gratings on PET using direct laser interference patterning

Recently, product protection and tracking became increasingly important due to the spread of piracy and counterfeiting. A common anti-counterfeiting procedure is embedding holographic motives or logos onto the good. If the motive is engraved directly onto the material surface, these features are inseparable from the good adding a higher degree of security. Holographic coloring is achieved by fabricating periodic surface structures, where the dimensions of the spatial periods lie in the order of the wavelengths contained in the visible spectrum. However, the fabrication of such periodic features directly on the product surface at high resolution and manufacturing speed is still challenging. Direct Laser Interference Patterning (DLIP) is an industrial compatible method with high processing flexibility which allows the structuring of holographic motives with high resolution and throughput. In this work, DLIP is employed to produce diffraction gratings with variable spatial periods and feature heights on a transparent PET substrate, which is a polymer commonly used for mass consumer goods and packaging. A numerical model based on the finite element method was used to restrict the gratings' geometrical parameters that maximize the diffraction efficiency in reflection mode before their fabrication. Then, using the design of experiment approach, the laser processing parameters (laser power, pulse-overlap, spatial period) were selected in order to maximize the experimental first-order diffraction intensity, measured with a photospectrometer. The results allow to find the optimum set of parameters to fabricate homogeneous gratings with a first-order reflected intensity up to 4 % of the light source intensity.