Transparent and flexible photonic crystal films with enhanced optical performance for multi-level anti-counterfeiting applications.

Self-assembly of colloidal particles in photocurable monomer provides a facile way to fabricate functional photonic crystal (PC) films. However, the inadequate optical performance of PC films limits their practical applications. In this work, we develop a new method to fabricate versatile PC films with enhanced optical performance assisted by solvent. The corresponding reflection intensity can reach 79% at the particle volume fraction of 45%, which is nearly 5 times higher than that of non-solvent-assisted PC films. Additionally, the maximum reflection intensity of solvent-assisted PC films is approximately twice that of non-solvent-assisted PC films. Meanwhile, the transmittance of the obtained PC films at the thickness of 94 μm remains above 85%, and the rubber-like matrix P(PEGPEA) further endows the PC films with high flexibility. This preparation process of PC films is straightforward and extensible in terms of colloidal particles, solvent and monomers. Based on the high-quality PC films, we fabricate multi-level anti-counterfeiting labels through the subsequent swelling of a second monomer, which can elicit distinct responses to water and achieve multi-level visual effects combined with transparency, flexibility and angle-dependent effects of the as-prepared PC films. This work offers a novel and simple method to fabricate functional PC films with enhanced optical performance by increasing the particle volume fractions and shows great potential in anti-counterfeiting applications.