Protorefractive Effect in GaP

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The purpose of this research is to measure the fundamental properties of GaP whose photorefractive effect was first discovered by us. We measured its photorefractive gain coefficient and time constant at room temperature. We used both a 633nm He-Ne laser and a 806nm laser diode. It was found that GaP could be used at shorter wavelengths than GaAs and InP, and that the photorefractive properties were similar to theirs. The experimental results agreed with the standard theory in which single carrier and single trap were assumed. Further we measured photorefractive properties at various temperatures. At about 150K, extraordinary behavior in photorefractive gain was observed and at almost the same temperature the photochromic effect was also detected. These effects are caused by the shallow traps which become effective at low temperatures. These experiments will be useful for investigating deep trap levels. As applications, we demonstrated the generation of phase conjugate wave using degenerate four wave mixing. The reflectivity of 0.3 % was obtained. Although the reflectivity is low, the response time is much shorter than the ferroelectric materials.