表层厚度渐变一维耦合腔光子晶体的反射相位特性及其应用

Reflection phase characteristics and their applications based on one-dimensional coupled-cavity photonic crystals with gradually changed thickness of surface layer

利用传输矩阵法研究了表层厚度渐变的一维非对称耦合腔光子晶体的反射相位特性.研究表明,光子禁带内(包括缺陷模附近)的反射率在98%以上,且基本不受表层厚度影响,特别是,在非正入射情况下,简并的缺陷模随着表层厚度的变化会发生分裂;进一步研究发现,在缺陷模分裂处附近,TE,TM偏振的反射相位以及它们之间的相位差均敏感地依赖于表层厚度的变化,从而使得反射光的偏振态也随表层厚度的变化而敏感变化,其物理机理在于缺陷模分裂所造成的剧烈相位变化.基于上述特性,设计了一种表层厚度呈二维周期变化的一维光子晶体结构,从该结构反射的激光经透镜聚焦后,在聚焦区域同时存在各种偏振态(包括沿不同方向的线偏振、左旋或右旋圆偏振、椭圆偏振等)的子光束,它们叠加后在聚焦区域将产生具有无规相位和无规偏振态的光场.以上结果能有效降低激光的相干性,在激光核聚变等领域有潜在的应用价值.

In this paper, we first improve the traditional transfer matrix method to adapt to one-dimensional photonic crystal consisting of arbitrary materials, and then use it to study the reflection phase characteristics of two kinds of photonic crystals, i.e., a simple periodic photonic crystal structure and a coupled-cavity asymmetric photonic crystal with gradually changed thickness of surface layer. For both of the structures, the reflectivity within photonic band gap is above 98%and hardly affected by the thickness of the surface layer. However, their reflection phases exhibit distinctly different properties. For the simple photonic crystal structure, the reflection phases of both TE and TM polarizations are sensitively dependent on the thickness of surface layer, but their phase difference is almost the same as the thickness of surface layer varies, which cannot change the polarization of reflected light. While for the coupled-cavity asymmetric photonic crystal structure, studies show that the degenerate defect modes within photonic band gap will split as the thickness of the surface layer varies. Moreover, around the splitting defect modes the reflection phases of both TE and TM polarizations, as well as their phase difference, are sensitively dependent on the thickness of surface layer, resulting in sensitive polarization change of reflected light. The physical reason is attributed to the dramatic phase change caused by the splitting of degenerate defect modes. The above reflection phase characteristics of coupled-cavity asymmetric photonic crystals have potential in lowering or even eliminating the coherence of lasers in some special application cases.As an example, we design a one-dimensional photonic crystal structure with two-dimensional periodic varying thickness of surface layer. After an oblique incident narrowband laser beam is reflected from this structure and then focused by a lens, various polarized light beams（including linear polarized light beams along different directions, left-hand（or right-hand） circular（or elliptical） polarized light beams） will exist simultaneously, whose superposition will produce optical field with random phase and polarizations in the focal region. These results can effectively reduce the coherence of lasers, which holds promise in many fields such as laser nuclear fusion.