2. Experimental
High quality of GTO crystal was grown by the Czochralski method. The growth details can be found elsewhere [13]. 2 mm thick wafers with the same size were cut from the as-grown crystals and were then annealed at 1400 1C for 24 h under air atmosphere and flowing hydrogen atmosphere. The luminescent spectra and decay curves were recorded on the Jobin Yvon Fluorolog-3 spectrometer. 450 W Xeon lamp and a pulsed laser (266 nm) by the fourth harmonic of a Nd:YAG laser were used as the excitation sources for the steady-state and time-resolved luminescent measurements, respectively. For time-resolved lumi- nescence, we used a photomultiplier with a variable load resistor and recorded the signal with a Tektronix TDS 2024 oscilloscope