Researchers at Ecole Polytechnique Fédérale de Lausanne have combined silver nanostructures with polarised light to yield a range of brilliant colours, which can be used to encode messages. The polarisations are used as keys and the message is encrypted in a quaternary colour subset.
https://onlinelibrary.wiley.com/doi/10.1002/adom.202202165
https://www.sciencedaily.com/releases/2023/02/230213120701.htm