Identification of interstitial oxygen color centers in CaGdAlO4 single crystals and their elimination strategy

Abstract

CaGdAlO₄ (CGA) crystals are promising gain media for ultrafast lasers due to their favorable thermal and optical properties. However, color center defects often introduce unwanted optical absorption, degrading laser performance. This study identifies interstitial oxygen atoms (O_i) as the dominant source of coloration in CGA crystals through a combination of X-ray photoelectron spectroscopy, optical absorption measurements, and density functional theory calculations. The remarkable consistency between the measured and calculated absorption spectra unequivocally identifies O_i as the primary defect responsible for coloration. The incorporation of O_i introduces mid-gap electronic states, leading to strong absorption in the 250–450 nm range and imparting a yellow color. We demonstrate that controlling the atmosphere during polycrystalline synthesis is crucial, i.e., using a reducing atmosphere yields white, defect-free polycrystalline powders, which subsequently enable the growth of high-quality, colorless CGA single crystals via the Czochralski method under a N₂/H₂ mixed atmosphere. This work clarifies the defect origin in CGA and provides an effective strategy for producing high-optical-quality crystals for advanced photonic applications.