High-efficiency infrared-to-visible upconversion of Er3+ in BaCl2

Abstract

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 μm laser diode with a power density of ∼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 Y_0.8Er_0.2F₃ is pasted. A 0.97 μm laser diode excitation on the chloride yielded blue (0.49 μm), green (0.55 μm), and red (0.66 μm) fluorescences, visually exhibited as a bright greenish‐white emission. The visible fluorescence excited by 0.8, 0.97, or 1.5 μm 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.