Asymmetric optical camouflage: tuneable reflective colour accompanied by the optical Janus effect

Abstract

Encryption: Nanostructured windows deliver colorful secrets Applications including anti-counterfeiting measures may benefit from an optical device that encrypts information as viewpoint-dependent colours. Recent studies have shown that embedding metal nanoparticles within transparent materials can produce structural colours that appear differently when seen from the front or back. Yong-Sang Ryu from the Korea Institute of Science and Technology in Seoul and co-workers have now developed a simplified technique for tuning color output from these smart windows. The researchers created optical cavities by sandwiching a porous insulator between a reflecting layer and a film of gold nanoparticles. Dramatic color changes from both viewing directions were achieved by altering the cavity's effective refractive index using surface modification and liquid infiltration. Engraved messages that were visible in air could be obscured on one side by submersing the windows into specific solvents. Going beyond an improved colour gamut, an asymmetric colour contrast, which depends on the viewing direction, and its ability to readily deliver information could create opportunities for a wide range of applications, such as next-generation optical switches, colour displays, and security features in anti-counterfeiting devices. Here, we propose a simple Fabry-Perot etalon architecture capable of generating viewing-direction-sensitive colour contrasts and encrypting pre-inscribed information upon immersion in particular solvents (optical camouflage). Based on the experimental verification of the theoretical modelling, we have discovered a completely new and exotic optical phenomenon involving a tuneable colour switch for viewing-direction-dependent information delivery, which we define as asymmetric optical camouflage.