CdF2 : YbF3 : ErF3—An Efficient Infrared to Visible Upconverting System

Abstract

Single crystals of doubly doped with and efficiently convert infrared radiation (∼0.93 μm) from a diode to red and green light. By varying the Yb concentration, phosphors have been made which produce a red or green visual response. For the series of crystal samples of studied: at concentrations below 3%, only red Er3+ emission peaks were observed (6450–6800Å); at 3% , red and green spectra (5300–5700Å) were observed, the green bands being much less intense; the most intense phosphor contained 10 mole percent (m/o) Yb3+, and the green intensity was an order of magnitude higher than the red. Charge compensation by of the rare earth ions in greatly reduces the intensity of emission bands, as well as the intensity of the absorption band at 976 nm, in agreement with previous studies of the absorption spectra properties of the system (1). Recent results of 19F NMR of single crystals of containing high concentrations of and, respectively, indicate that most of the rare earth ions and their associated interstitial fluorides are found in dimers in the crystal (2). The intense absorption band observed at 976 nm in Yb3+‐doped can be assigned to this dimer, which is destroyed by the use of Na+ as a charge compensator in place of interstitial fluoride, the native charge compensating defect. Similarly, as the accepted mechanism of the infrared‐to‐visible conversion in the Yb3+ sensitized Er3+ activated systems involves the transfer of two infrared photons from Yb3+ to Er3+, phosphor emission by diode excitation will decrease when the complex is destroyed by substitution. The 976 nm strong absorption band was also observed in samples (10% Yb, 1% Er) prepared with substituted for without appreciable decrease in the intensity; however the emission intensities of both green and red bands were significantly enhanced for all of the samples codoped with . There appears to be a plateau for the compositions 1.5–3% and a decrease in the 4.5 and 10% samples. Of all the materials investigated to date, and codoped with Yb3+ and Er3+ are reported to be the most efficient upconverting systems with a quantum efficiency between 10−4 and 10−3 . The quantum efficiency of is found to be 0.23% and that of , 0.70%. These values were obtained under intense excitation with a diode. Lower values might be found at lower excitation densities.