Summary form only given. The formation of space charge fields on the vicinity of a free surface of a crystal is a topic of growing importance. The piezo-electric grating is a result of the space charge field created by charge separation on the surface of the crystal. We report on the detection of piezoelectric and thermal surface relief gratings. Experiments were performed with a 3 mm thick [110] cut BSO crystal. Expanded Ar-ion lasers beams (A=457 nm) interfere at the front surface of the crystal. The crystal sample possess a large absorption coefficient at this wavelength so the reflected beams from the back surface are negligible.
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