HIGH-EFFICIENCY INFRARED-TO-VISIBLE UP-CONVERSION OF ER-3+ IN BACL2

Highly efficient infrared-to-visible upconversion has been observed in Er3+-doped BaCl2 phosphors. The composition optimized for maximum green emission was around 25ErCl3 (mol %), which contains 5-15 mol % more active Er3+ ions than those in the conventional Er3+-doped fluoride phosphors. Pumped by a 0.8 mum laser diode with a power density of approximately 3 W/cm2, chloride (25ErCl3-75BaCl2) shows a very bright green emission with the intensity being two orders of magnitude larger than that of the commercially available IR sensor card on which an optimized fluoride phosphor Y0.8Er0.2F3 is pasted. A 0.97 mum laser diode excitation on the chloride yielded blue (0.49 mum), green (0.55 mum), and red (0.66 mum) fluorescences, visually exhibited as a bright greenish-white emission. The visible fluorescence excited by 0.8, 0.97, or 1.5 mum laser diodes shows quadratic or cubic dependencies on the excitation power over the entire power range for the chlorides but lower dependences for the fluoride. The differences in the upconversion characteristics between the chlorides and fluoride are discussed in terms of the rate equations and attributed principally to the different Er3+ ... Er3+ interionic energy transfer probability and the different multiphonon decay rate from the excited states of Er3+ in these matrices.