Influence of air annealing temperature and time on the optical properties of Yb:YAG single crystal grown by HDS method

Abstract

8at.% Yb:YAG plate single crystal with the dimension of 170mm×150mm×30mm was grown in vacuum by Horizontal Directional Solidification method. Aimed at blue-green color centers, annealing treatments of 15mm×15mm×1mm samples from 900°C to 1400°C for 5h and at 900°C from 5h to 40h in air were conducted. The absorption spectra, emission spectra, fluorescence lifetime and X-ray photoelectron spectroscopy of samples under different annealing conditions were measured at room temperature, respectively. Annealing at above 1000°C for 5h or at 900°C for 40h made the blue-green color centers disappear and the samples turned to transparent. Absorption coefficients decreased in the 300nm–800nm wavelength range, emission intensities increased and emission bands broadened around 486nm and 1029nm with increasing temperature up to 1200°C, then varied inversely. These values decreased or increased monotonically with increasing annealing time at 900°C. The maximal increases of fluorescence lifetime were 62.3% and 64.7%, respectively. The calculated emission cross section of 1200°C for 5h was up to 4.4×10−20cm2. In X-ray photoelectron spectroscopy, the concentrations of oxygen vacancies reduced from 1.28% down to absence by annealing. These experiments show that color centers are detrimental to the optical properties of HDS-Yb:YAG laser crystal and optimal annealing treatments should be conducted.