Abstract

New security devices based on innovative technologies and ideas are essential in order to limit counterfeiting’s profound impact on our economy and society. Interference security image structures have been in circulation for more than 20 years, but commercially available iridescent products now represent a potential threat. Therefore, the introduction of active materials, such as electrochromic ${\mathrm{WO}}_3$, to present-day optical security devices offers interesting possibilities. We have previously proposed electrochromic interference filters based on porous and dense ${\mathrm{WO}}_3$, which possessed an angle-dependent and voltage-driven color shift. However, the low index contrast required filters with a high number of layers. In this article, we increase the index contrast (0.61) by mixing ${\mathrm{WO}}_3$ with ${\mathrm{SiO}}_2$ and study the physical and electrochromic properties of mixtures. We next combine high and low index films in tandem configurations to observe the bleaching/coloration dynamics. To account for the film performance, we propose a simple explanation based on the differences in electron diffusion coefficients. An 11 layer electrochromic interference filter (EIF) based on the alternation of pure ${\mathrm{WO}}_3$ and ${({\mathrm{WO}}_3)}_{0.17}{({\mathrm{SiO}}_2)}_{0.83}$ films with a blue to purple angular color shift is then presented. Finally, we discuss possible applications of these EIFs for security.

©2012 Optical Society of America

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