Anti-counterfeiting holographic liquid crystal gels with color and pattern control

Abstract

Reflective gratings based on holographically patterned liquid crystals exhibit electrically switchable color due to changes in the periodic refractive index of the material. Previous work has shown switching behavior between reflective and transparent states or between transparent and scattering states. Here, we report for the first time a novel anti-counterfeiting material for optical security and encryption using holographic polymer stabilized liquid crystal gels (H-PSLC gels) with a relatively small polymer concentration (5–20 wt%). The discrete reflection color of H-PSLC is induced by applying an alternating current (AC) electric field, and the position of the AC field induced reflection notch can be tuned by the subsequent application of a direct current (DC) field. The holographic notch of the H-PSLC exhibits a large reversible red-shift (Δλ ≈ 200 nm) with high reflection efficiency (60–80%). The dynamic red-shift of the holographic reflection corresponds to gradients in the holographic pitch caused by the deformation of the polymer network under a DC field. Simultaneous photopatterning and holographic photopolymerization produce an initially transparent LC gel that can electrically induce a variety of reflective colors, images, and diffraction patterns. The electrically controllable color, image and pattern of H-PSLC gels can be used in anti-counterfeiting materials for optical security and encryption.