Bio-inspired multilayer polarizer for anti-counterfeiting

Poncelet, Olivier [UCL] Simonis, Priscillia [PMR, Université de namur/Belgique] Francis, laurent [UCL]

In this work, we investigate a new way to secure anti-­counterfeiting using light polarization effects inspired from natural photonic structures found on the scales of Papilio butterflies (blumei, ulyss) and on cuticles of Cicendela (chinensis). Polarization effects is only observed by the human eye helped by a polarizer. Thus it can insure two of the three levels of security in banknotes and passports [1]. The first level is a pattern easily recognized by anyone as a colorized symbol. The second level is a pattern that requires a simple device, i.e. a UV lamp or a polarizer, to be observed. The optical structure of papilio and cicendela consists in multilayered cavities. It allows back-­‐reflections by multiple reflections near Brewster angle which result in polarized spectrum [2]. Moreover, geometrical effect induces rotation of the polarization state [3] so that the light is not blocked by cross-­polarizer (Fig. 1.a). These effects are canceled from the macroscopic point of view due to spherical symmetry of the structure. Breaking the symmetry is thus mandatory. For this purpose, we reproduce the photonic crystal using microfabrication techniques in cylindrical symmetry with two orthogonal directions. Bragg mirror was obtained by atomic layer deposition (ALD). Theoretical reflection spectrum was computed by multiscale simulations (ray-­tracing and transfert matrix) and compared with FDTD. Characterizations made by optical microscope and in situ polarizer show that the performed structure enhances polarization effects even at macroscopic point of view (fig. 1.b). Polarization effects have never been used so far in anti-‐counterfeiting industry. This work shows promising results on this way.