Up-Conversion Photoluminescence of Sol-Gel Derived CaY₂O₄ Powders Under 980 nm Excitation

Up-conversion is an anti-stokes process that can convert near-infrared light into visible light. Besides the requirement of high optical conversion efficiency, the thermochemical stability of the host materials is critical for in practical applications, and a stable oxide host material is optimal. CaY₂O₄ follows the same ordered structure of CaFe₂O₄, which is composed of an (R₂O₄)^2- (R = rare earth metal) framework of double octahedral moieties with rare earth ions residing within the framework. CaY₂O₄ is a promising host material due to its favorable characteristics such as high chemical and thermal stability, low-phonon energy, and environment friendliness. Rare earth ion (Er³⁺ and Yb³⁺)-doped nano-crystalline phosphors were prepared by a sol-gel process. The synthesized sample was characterized by X-ray diffraction analysis and the crystallite size was confirmed by Scherrer's formula and transmission electron microscopy. The surface morphology of the powders was determined by scanning electron microscopy. X-ray diffraction patterns confirmed their pure orthorhombic structure after annealing at 1,200 °C and the morphology of particles was found to be a nearly spherical shape with a diameter of the order of ~100 nm. Photoluminescence properties of the powders were measured by exciting the samples with a 980 nm diode laser at room temperature. Under the 980-nm laser excitation, the green and red up-conversion emissions were observed at around 520- 540 nm, 540-570 nm and 640-680 nm, which, are attributed to the transitions of ²H_11/2 →⁴I_15/2, ⁴S_3/2 →⁴I_15/2 and ⁴F_9/2 →⁴I_15/2 of Er³⁺ ions, respectively. The up-conversion intensity as a function of laser power shows that the up-conversion mechanism corresponding to green and red emissions occurs via a two-photon process.