Relaxation of ultraviolet-radiation-induced structure and long-lasting phosphorescence in Eu 2+ -doped strontium alumino
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Y. Shimizugawa Department of Optical Materials, Osaka National Research Institute, Osaka 563-0026, Japan
K. Kojima Faculty of Science and Engineering, Ritsumeikan University, Shiga 525-8577, Japan
K. Tanaka and K. Hirao Department of Material Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 606-8501, Japan (Received 9 August 2000; accepted 18 September 2000)
We report on the relaxation of ultraviolet-radiation-induced structure and long-lasting phosphorescence in Eu2+-doped glass samples with compositions of xAl2O3 ⭈ 40SrO ⭈ (60 − x)SiO2 ⭈ 0.05Eu2O2 ⭈ 0.05Dy2O3 (x ⳱ 0, 10, 30) (mol%). After irradiation by an ultraviolet lamp (max ⳱ 254nm) with a power density of 5 mW/cm2 for 30 min, a visible light (peaking at 510 nm) could be seen with the unaided eye in the dark even 24 h after the removal of the activating light. Thermostimulated luminescence glow curves, x-ray absorption, and electron-spin-resonance spectra were measured. We suggest that the long-lasting phosphorescence in the Eu2+-doped glass samples resulted from the recombination of electrons and holes at shallow traps in the glass matrix that can be thermally released at room temperature, and energy transfer between the recombination centers and Eu2+ ions.
I. INTRODUCTION 1
In fiber gratings and photostimulated luminescence,2–5 defects artificially introduced by using external radiations such as ultraviolet (UV) light and x-rays are important. In the formation of Bragg fiber gratings, the increase in refractive index of Ge-doped silica glasses after the irradiation by UV light is suggested to arise from the formation of various color centers (e.g., GeE⬘, which is an electron trapped in an sp3 orbital of germanium at the site of oxygen vacancy6) and local densification of the glasses.7 Fiber gratings can be used as optical filters, which can be directly connected to optical communication systems. In the case of photostimulated luminescence, defect centers (e.g., halogen ion deficit sites) act as electron trapping centers.2–5 The electron trapped in the F center, after x-ray irradiation, is released by excitation of a visible or an infrared laser, and subsequently recombines with a trapped hole, leading to emitted light whose intensity is proportional to
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II. EXPERIMENTAL
Compositions of the glass samples used in this study were 40SrO ⭈ xAl 2 O 3 ⭈ (60 − x)SiO 2 ⭈ 0.05Eu 2 O 2 ⭈ 0.05Dy2O3 (x⳱ 0, 10, 30) (designated as xED) (mol%) and 40SrO ⭈ 30Al2O3 ⭈ 30SiO2 ⭈ 0.05Eu2O2 (designated
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the number of F centers, which in samples is proportional to the x-ray dose.2–5 Photostimulable luminescent phosphors have been used as materials for two-dimensional x-ray sensors.2,3 Long-lasting phosphorescence is also a radiationinduced defect-related phenomenon.8 Recently, longlasting phosphorescence phenomenan have been observed in various rare-earth-doped crystals and glasses induced by UV light and infrared femtosecond laser.9–13 In this ar
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