Effect of host on the radiative (upconversion emission) as well as non-radiative relaxation (laser induced optical heating) in Tm3+/Yb3+ co-doped phosphors

Abstract

Tm3+/Yb3+ co-doped YVO4 (YVO:Tm, Yb); GdVO4 (GVO:Tm, Yb); Y2O3 (YO:Tm, Yb); Gd2O3 (GdO:Tm, Yb) and CaAl2O4 (CAO:Tm, Yb) phosphor samples have been synthesized using high temperature Solid State Reaction method. X-ray diffraction measurements confirm the pure phase formation of all the five phosphors. Fourier Transform Infrared (FTIR) measurements give information about the different vibrational groups and also the phonon frequency of the host materials. The radiative as well as non-radiative phenomena are observed in all phosphors on excitation with 980 nm laser radiation. Radiative process gives upconversion (UC) emission in Visible and NIR regions due to Tm3+ ion. On the other hand the non-radiative processes generate heat in the material known as laser induced optical heating. The Upconversion (UC) emission behavior of all the phosphors has been investigated on 980 nm excitation which gives intense blue, weak red and prominent NIR emissions. The overall UC emission intensity is maximum in Tm3+/Yb3+ co-doped GdVO4 phosphor as compared to the other four. The crystalline behavior, surface morphology and vibrational behavior play important role in emissive properties. Laser induced optical heating is observed in all the phosphors which show an increase in heating in the phosphors with the increase in pump power. The optical heating is affected by phonon frequencies of the phosphor materials. In the present work, heating is more prominent in the case of vanadate host (a self-activated host) which has phonon frequency near about 800 cm−1. Overall, the UC emission and laser induced optical heating is dominant in self-activated vanadate host as compared to the other oxide hosts. In between vanadate hosts, GVO:Tm, Yb shows better UC emission as well as laser induced optical heating.