| Photorefractive polymer was a new nonlinear optical material beginning to study from the end of twentieth century. Its characteristic was that refractive index would become continuous and periodic changing when light was irradiated it. According to defining of photonic crystal, the photonic band gap should exist in material when refractive index changed periodic. The holographic grating using photorefractive polymer had periodic dielectric structure, so it could be regard as photonic crystal. This method to prepare photonic crystal was more easy and simple than conventional method.In the letter, we discussed this content mainly. The letter was divided into two parts: one part was the theory of calculation, another was experiment test. In the first part, we discussed about the theory of photonic band gap. We knew the refractive index of photorefractive polymer was continuous and periodic changing, and we regarded a sin wavelength as a period. It was calculated photonic band gap of photorefractive material by finite difference time domain method. It was indicated that the band gap existed in photorefractive material. The position was affected by refractive index and the length of period. The deepness was deeply affected by the number of period. The deepness difference was about 27% when the number of period was 300 and 500 respectively. The number of band gap was affected by refractive-index modulation amplitude.In the second part, we discussed about preparation of sample. Photorefractive polymer mixing azo compound was the best photorefractive polymer material. We mainly prepared such material to test photonic band gap. So we could be testified our calculation. We prepared two azo compounds by diazo couping reaction in experiment. The compounds was analyzed and characterized by IR and UV test method. The result indicated azo in our compound. Azo absorbing band appeared in 418nm and 432nm. At last, we prepared film sample mixing PMMA. This supplied sample for optical testing in future.
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