基于超晶格结构衍射图的倒易矢量分布

Distribution of Reciprocal Vectors Based on Diffraction Patterns of Superlattice Structures

根据超晶格结构的激光衍射图,提出了一种定量确定倒易矢量分布的实验方法.首先,将正方形周期超晶格结构作为参考光栅,得到其衍射图.根据傅里叶光学理论,计算出基本倒易矢量的大小,与衍射图上的几何长度建立标尺关系.通过引入矩形超晶格结构,证明了该方法在周期超晶格结构中的可行性.其次,将H型和谢尔宾斯基分形超晶格结构作为光栅,获得的衍射图与正方形结构衍射图进行对比.由衍射点间的几何长度比值,推算出分形衍射图中的倒易矢量分布.根据倒易矢量和准相位匹配谐频的基频波长之间的定量关系,理论计算出能够进行的谐频波长.最后,实验制备分形结构LiNbO3非线性光子晶体,探测准相位匹配倍频,所实现的倍频波长与理论计算值相吻合.谢尔宾斯基分形结构光栅在理论与实验上均可实现1.352μm的有效倍频输出.

A experimental method was provided, in which the distribution of reciprocal vectors can be easily obtained by their diffraction patterns. First, the diffraction pattern of square periodic superlattice as a reference grating was gotten. The value of the reciprocal vector according to the Fourier optics was calculated, and the scale relation with the geometric length in the pattern was built. By introducing the rectangular superlattice structure, this method was proved to be right in the periodic superlattices. Secondly, the diffraction patterns of the H-shape and Sierpinski fractal superlattice structures were realized and made a comparison with the square structure. The reciprocal vectors in two structures could he calculated based on the obtained geometric length ratio. Then by quantitative relation between the reciprocal vectors and fundamental wavelengths in quasi-phase matching processes, the harmonic wavelengths were calculated. Finally, the LiNbO3 nonlinear photonic crystals with fractal superlattice structures were fabricated experimentally. It can be gotten that the experimental quasi-phase matching harmonic wavelengths agree with the calculated ones. Especially, for Sierpinski fractal superlattice, by calculation, the effective second harmonic of 1. 352μm can be realized. And the corresponding results can be accomplished by experiments.