A standard check for counterfeits is viewing bank notes under ultraviolet or infra-red light. This causes invisible inks to glow visibly and is one of the proven tricks in optical document security. Microprint is another method used as an anti-counterfeiting tool. As the name indicates, microprints conceal information on documents as they are extremely small for the eye to see. However, microprints and invisible inks frequently only display a single color and are separate elements.
Recently, researchers from the Singapore University of Technology and Design (SUTD) have described a plasmonic upconversion optical security device, which displays an ultrahigh resolution color print under white light while exposing diverse upconversion luminescent information under infrared illumination. The new optical security devices have potential applications in discouraging counterfeiting of critical documents and packages of high-value medicines.
Principal researcher, SUTD Associate Professor Joel Yang, terms it "TRUE color printing", where "TRUE" stands for "Tunable-Resonator Upconverted Emission". A monolayer of upconversion nanophosphors (NaGdF4:Yb) was self-assembled within a 15 nm gap between aluminum disks and a continuous aluminum film. The robust electromagnetic fields restricted within the metal-insulator-metal gap boosts the brightness and of the nanophosphor emitters by two orders of magnitude. Remarkably, in this TRUE color printing, a variety of luminescent colors were accomplished with one type of upconversion nanophosphors under one excitation source. Typically, doping with varied lanthanide elements or using several excitation lasers are required to attain many luminescent colors. Rather, the interaction between these nanophosphors and their local setting causes them to shine with various colors.
Present-day optical security devices are typically one dimensional and merely display a set of encrypted information. While in TRUE color printing, both amplitude of the white light and upconversion luminescence are concurrently used to encode the information. Of late, it is well known that various sizes of plasmonic resonators reflect various colors under white light. In the meantime, they will trigger different energy transfer channels of upconversion nanophosphors under laser excitation, releasing various luminescent colors. Thus, the white light colors and luminescent colors were both used to engineer the optical security devices, which exhibit a colored butterfly while exposing a luminescent butterfly. To increase the level of confidentiality, upconversion emitters are precisely integrated into particular positions, while leaving other areas non-luminescent. A visible colorful butterfly was revealed under white light while a football and a few English letters "UCNP" was recovered using laser illumination. The different display information under white light and laser source make TRUE color printing extensively applicable in anti-counterfeiting such as banknotes, passport, and ID cards.