Persistent Room-Temperature Photodarkening in Cu-Doped β-Ga_{2}O_{3}

β-Ga_{2}O_{3} is an ultrawide band gap semiconductor with emerging applications in power electronics. The introduction of acceptor dopants yields semi-insulating substrates necessary for thin-film devices. In the present work, exposure of Cu-doped β-Ga_{2}O_{3} to UV light >4  eV is shown to cause large, persistent photo-induced darkening at room temperature. Electron paramagnetic resonance spectroscopy indicates that light exposure converts Cu^{2+} to Cu^{3+}, a rare oxidation state that is responsible for the optical absorption. The photodarkening is accompanied by the appearance of O─H vibrational modes in the infrared spectrum. Hybrid function calculations show that Cu acceptors can favorably complex with hydrogen donors incorporated as interstitial (H_{i}) or substitutional (H_{O}) defects. When Cu_{Ga}-H_{O} complexes absorb light, hydrogen is released, contributing to the observed Cu^{3+} species and O─H modes.