Afterglow (i.e. persistent luminescence) is a self-sustained luminescence phenomenon whereby a material (i.e. persistent phosphor) glows in the dark for hours after the excitation light source has been switched off. Nowadays persistent phosphors emitting in the visible and near-infrared spectral ranges are being used in daily life, e.g., security signs, emergency route signs, traffic signage, dials and displays, and medical research. However, the widespread use of persistent phosphors is greatly hindered because of the need of high-energy excitation light, mostly ultraviolet light, to charge the materials. Therefore, solutions to achieving effective excitation with low-energy light, particularly the visible light, are in demand. This project investigates a new charging process that enables low-energy visible-light sources (e.g. laser diodes) to be as effective as the commonly used high-energy ultraviolet light sources (e.g. ultraviolet lamps) in charging persistent phosphors. The outcomes of this research have a potential to open new strategies for studying and utilizing persistent phosphors. The project provides an interdisciplinary training experience to graduate, undergraduate and K-12 students in luminescent materials research and applications. The undergraduate and K-12 education is implemented through the existing programs at University of Georgia, such as the NSF-sponsored Peach State-Louis Stokes Alliance for Minority Participation program, NSF-sponsored Research Experiences for Undergraduates sites, and Young Dawgs program (for K-12 education).
It is a general knowledge in persistent luminescence that high-energy ultraviolet light is usually necessary in order to effectively charge a persistent phosphor. However, the need of high-energy excitation compromises some applications where the ultraviolet light is unavailable or unsuitable. This project aims to tackle this limitation by investigating a new, two-photon up-conversion charging (UCC) process where low-energy visible-light sources (e.g. laser diodes) are as effective as the commonly used high-energy ultraviolet light sources (e.g. ultraviolet lamps) in charging persistent phosphors. In the UCC concept, two visible photons from a visible-light laser diode are successively absorbed by a UCC enabling ion (e.g. trivalent chromium ion) so that the high-energy delocalization state of the ion is reached and the electron traps in the phosphor are filled. The UCC enabling ions include trivalent chromium, divalent manganese, quadrivalent manganese, trivalent praseodymium, and trivalent neodymium. Several power-tunable visible-light laser diodes with wavelengths in the range of 400-700 nm are used as the excitation sources. New optical measurement techniques for UCC, such as UCC excitation spectroscopy, are under development in order to acquire spectral data that are essential for understanding the energy absorption, electron transfer, electron trapping and de-trapping processes involved in the UCC.
