Abstract

Up to now, most of the experimental examples involving the anti-Stokes fluorescence called up-conversion were dealing with rare earth ions [1]. For the first time we have found that tetravalent uranium as a doping ion in single crystal as well as in microcrystalline powder form of thorium tetrahalides acts as an up-converter. In the case of ThBr₄ : U⁴⁺, infrared excitation with λ > 0.82 µ at room temperature gives an emission band in the red between 0.67 and 0.72 µm |2|. The power law of the red emission with respect to the infrared excitation is found to be quadratic ; this is the evidence that excitation is obtained by a two-photon process. Furthermore, we found that the 0.69 µm emission follows a linear law with the U⁴⁺ concentration for a concentration range from 0.006 % up to 0.05 % in weight. From this result, we can conclude that the up-conversion process takes place in one uranium ion. For higher concentration a quenching effect is observed. We found that the efficiency was optimized using microcrystalline powder with grain size of about 0.2-0.3 µm and a concentration of 0.05 %. In this case the efficiency is multiplied by a factor of 20 in comparison with the same sample single crystal. This is believed to be due to photon trapping as demonstrated by life time variations with sample shapes. The red emission from ³P₀ is obtained by the absorption of photons at 0.95 µm ³H₄ → ³H₆ and 1.17 µm ³H₄ → ³F₆ as shown by the double beam [1] excitation spectra of figure 1. The level diagram (fig. 2) of U⁴⁺ shows that the ³P₀ fluorescence can be obtained by different ways such as summation of two photons (0.95 µm plus 1.17 µm or 2 × 0.95 µm or 2 × 1.17 µm) with phonon assistance, followed by a multiphonon relaxation.

© 1984 Optical Society of America