Viewing bank notes under ultraviolet or infra-red light is a common check for counterfeits. Doing so causes invisible inks to glow visibly and is one of the most tried and tested tricks in optical document security. Microprint is another technique used as an anti-counterfeiting tool. As the name suggests, microprints hide information on documents because they are too small for the eye to see. However, microprints and invisible inks often only exhibit a single color and are separate elements. Scientists from the Singapore University of Technology and Design (SUTD) have recently reported a plasmonic upconversion optical security device, which displays an ultrahigh resolution color print under white light while revealing different upconversion luminescent information under infrared illumination. The presented optical security devices have potential applications in deterring counterfeiting of important documents and packages of high-value medicines.
You can find more details at https://www.sutd.edu.sg/Research/Research-News/2019/5/Optical-security-Tunable-resonator-upconverted-emi