Optical security: Tunable-resonator upconverted emission color printing

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.

Principal researcher, SUTD Associate Professor Joel Yang, calls it "TRUE color printing," where "TRUE" stands for "Tunable-Resonator Upconverted Emission." A monolayer of upconversion nanophosphors (NaGdF4:Yb) were self-assembled within a 15 nm gap between aluminum disks and a continuous aluminum film. The strong electromagnetic fields confined within the metal-insulator-metal gap increases the brightness and of the nanophosphor emitters by two orders of magnitude. Interestingly, in this TRUE color printing, a range of luminescent colors were achieved with one type of upconversion nanophosphors under a single excitation source. Usually, doping with different lanthanide elements or employing multiple excitation lasers are required to achieve multiple luminescent colors. Instead, the interaction between these nanophosphors and their local environment causes them to shine with different colors.

Current optical security devices are mostly one dimensional and only display a set of encrypted information. While in TRUE color printing, both amplitude of the white light and upconversion luminescence are simultaneously employed to encode the information. It is recently well known that different sizes of plasmonic resonators reflect different colors under white light. Meanwhile, they will activate different energy transfer channels of upconversion nanophosphors under laser excitation, emitting different luminescent colors. Therefore, the white light colors and luminescent colors were both employed to design the optical security devices, which display a colored butterfly while revealing a luminescent butterfly. To increase the level of secrecy, upconversion emitters are precisely incorporated in specific positions, while leaving other places non-luminescent. A visible colorful butterfly was displayed under white light while a football and some English letters "UCNP" was recovered with laser illumination. The different display information under white light and laser source make TRUE color printing widely applicable in anti-counterfeiting such as passport, banknotes, and ID cards.