Asymmetric optical camouflage: Tuneable reflective color accompanied by optical Janus effect

In modern optics, a variety of nanoscale materials and their localisation have been examined, as they lead to novel optical effects. Viewing direction sensitive information display utilising optical Janus effect has attracted great attention owing to its dynamic operation scheme which delivers discriminative information delivery. However, the integration of nano-materials within multiple layer limit their application in dynamic and real-time colour tuning.

In a new paper published in Light Science & Applications, a team of scientists, led by senior researcher Yong-Sang Ryu (Sensor System Research Center, Korea Institute of Science and Technology, Republic of Korea) & Professor Seung-Yeol Lee (School of Electronics Engineering, Kyungpook National University, Republic of Korea) and co-workers developed a liquid-permeable translucent mirror to achieve asymmetric reflective colour contrast depending on the viewing direction. Based on the simple and cost-effective metal-dielectric-metal (MDM) films, they designed an optical device which is capable of displaying distinct colours as well as messages with respect to the viewing directions. This optical device is comprised nanoscale air voids in the dielectric film space which plays a key role in colour tuning via an infiltration of exposed liquids into the optical hotspots. Because the physicochemical properties of the dielectric films lead colour creations in visible ranges, based on the thorough understandings of optical phenomena depending on the comprising nanostructures of MDM films, reflective colour tuning in varying refractive index of surrounding solvents were realized. These scientists summarize the optical properties of their device:

“We describe the design of a transreflective etalon that produces directionally asymmetric reflective colour depending on the direction of incident light. And we discovered that the precise control of the physicochemical properties of the comprising films led to creation of viewing-direction sensitive asymmetric information encryption varying both of nanostructures and surrounding liquids.

These results suggest that the manipulation of film geometries enables not only to display the colour contrast but to hide viewing-direction-sensitive message encryption via optical camouflage phenomenon.”

“Moving forward, bidirectional display of two different tuneable messages/images appear to be a tangible goal in the next step to allow for a wider range of photonic applications, including dynamic/informative colour filters, smart windows, optical switches, double-side colour displays, optical data storage devices, and anti-counterfeiting devices. Furthermore, as the proposed fabrication process is compatible with large-area substrates, the design can be applied to optical-sensing platforms” the scientists forecast.

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