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We have observed large second-order nonlinear optical response from zinc oxide (ZnO) thin films deposited on sapphire substrates by pulsed laser ablation. By comparing the second harmonic signal generated in a series of ZnO films with different crystallinity and thickness, we conclude that a significant part of the second harmonic signal is generated at the grain boundaries and interfaces.
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Cao et al. (1998) studied this question.
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