Upconversion blue emission dependence on the pump mechanism for Tm~(3+)-heavy-doped NaY(WO_4)_2 crystal

摘要

Upconversion blue emissions of Tm~(3+)-ion-heavy-doped NaY(WO_4)_2 crystals are investigated with three different near-infrared pump mechanisms. The dependence of upconversion efficiency on the pump mechanism is analyzed from the scope of the concentration quenching effect. Three cross-relaxation processes, ~3H_5 + ~1G_4 → ~3H_6 + ~1D_2, ~1G_4 + ~3H_6 → ~3F_4 + ~3F_3, and ~1G_4 + ~3H_6 → ~3F_3+ ~3F_4, which influence the upconversion dominantly in the Tm~(3+)-heavy-doped system, are demonstrated theoretically and experimentally. The results indicate that Yb~(3+) ions can weaken the concentration quenching effect of Tm~(3+) ions significantly so that the blue emission efficiency can be enhanced by one order of magnitude. At the same time, the wavelength of the pump source also has considerable influence on both the population of some crucial energy levels and the upconversion mechanism. Experiments show that the upconversion blue emission in Tm~(3+)/Yb~(3+) co-doped NaY(WO_4)_2 crystal under 980-nm laser diode excitation is the most intensive of these three different near-infrared pump mechanisms. The conclusions are confirmed by spectra measurements and calculations of Judd-Ofelt theory and Miyakawa-Dexter theory.