Spectroscopic Study of New, Effective Up-Conversion Charging Process in Persistent Phosphors

非技术描述：余辉（即持续发光）是一种自我持续发光现象，即在激发光源关闭后，材料（即持续荧光粉）在黑暗中发光数小时。如今，在可见光和近红外光谱范围内发射的持久性荧光粉被用于日常生活中，例如安全标志，紧急路线标志，交通标志，拨号和显示器以及医学研究。然而，由于需要高能激发光（主要是紫外光）对材料进行充电，持久性荧光粉的广泛使用受到很大阻碍。因此，需要用低能量光，特别是可见光实现有效激发的解决方案。本项目研究了一种新的充电工艺，使低能量的可见光源（如激光二极管）与常用的高能紫外线光源（如紫外光灯）一样有效地为持久性荧光粉充电。本研究的结果有可能为研究和利用持久性荧光粉开辟新的策略。该项目为研究生、本科生和K-12学生提供了发光材料研究和应用的跨学科培训经验。本科和K-12教育是通过乔治亚大学现有的项目实施的，比如美国国家科学基金会资助的桃州-路易斯·斯托克斯少数民族参与联盟项目、美国国家科学基金会资助的本科生研究经验网站和年轻的Dawgs项目（用于K-12教育）。技术说明：在持续发光中，为了有效地给持续荧光粉充电，通常需要高能紫外线，这是一个常识。然而，对高能激发的需求危及了一些无法获得或不适合紫外线的应用。该项目旨在通过研究一种新的双光子上转换充电（UCC）工艺来解决这一限制，其中低能量可见光源（例如激光二极管）与常用的高能紫外线光源（例如紫外线灯）在充电持久性荧光粉方面同样有效。在UCC概念中，来自可见光激光二极管的两个可见光子相继被UCC使能离子（如三价铬离子）吸收，从而达到离子的高能离域状态，并填充荧光体中的电子陷阱。UCC使能离子包括三价铬、二价锰、四价锰、三价镨和三价钕。采用波长在400 ~ 700 nm范围内的几个功率可调可见光激光二极管作为激发源。新的UCC光学测量技术，如UCC激发光谱，正在开发中，以获得光谱数据，这对理解UCC的能量吸收、电子转移、电子捕获和释放过程至关重要。

Non-technical Description: 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).Technical Description: 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.