Abstract
Electron paramagnetic resonance (EPR) spectra of Fe3+ in cubic symmetry and of Fe3+ at Si4+ site in axial symmetry due to oxygen vacancy have been studied and analyzed. The EPR spectrum of the home grown BSO:Fe3+ is shown in Fig. 1. The spectrum was recorded with a Braker ESP-300E spectrometer. With a view to understand the possible role of Fe3+ impurity in BSO on its photorefractive properties we have monitored the change in the intensities of EPR lines under He-Ne laser illumination. The measurements were done insitu at room temperature using optical transmission cavity. Under uniform illumination no detectable change in the intensity of the EPR line was observed. A grid of black lines 1 mm wide separated by 1 mm were drawn using a marker on the BSO crystal which was then illuminated by the laser at room temperature. The intensity of the signal went down under the illumination. The decrease in the intensity of the EPR signal when the laser is on, can be seen in Fig. 2. The decay of the signal when the laser is on and its growth when it is off are shown in Fig. 3. The signal were recorded by monitoring the peak intensity of the EPR signal of Fe3+. The decay constant using Fe3+ at cubic site is larger than that at noncubic site. This indicates that when the laser is on, the number density of Fe3+ goes down, while it goes up when the laser is off. Thus in a photorefractive grating in BSO, the bright regions have probably more Fe4+ centers and the dark regions have more Fe3+ centers. The long range nature of the Coulomb force in BSO helps to decrease the erasure rate of the grating with smaller fringe spacing. Thus in the present experiment when BSO is illuminated nonuniformly with a wide grating formation it has been possible to observe the photoinduced charge transfer in Fe3+.
© 1994 Optical Society of America
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