结合等离子金属光栅原理和多层膜理论，提出一种金属介质金属多层光栅结构以增强透射光的聚焦效应。利用时域有限差分(FDTD)法对该多层结构的聚焦特性进行模拟仿真，得到多层结构的聚焦特性随金属介质膜层数和凹槽数目的变化特性。结果表明，通过控制金属介质层层数或者凹槽数目，焦距明显改善，焦距可达9.15 \mm，最大焦深约为26.38 \mm，相对于传统金属光栅，聚焦效应得到了明显增强。

Based on plasmonic metal grating mechanism and multilayer theory, a subwavelength slit with metal-dielectric multilayer composite surface grating which can enhance focusing effect is proposed. According to the finite difference-time domain (FDTD) method, the numerical simulation results demonstrate that the focusing behaviors of the designed structure is related to the numbers of metal-dielectric layers and grooves. It is found that the focal length can be increased significantly by controlling the number of metal-dielectric layers. The maximum focal length and the maximum focus depth can reach 9.15 and 26.38 \mm, respectively. Therefore, the focusing performances are enhanced obviously compared with conventional mono-metal gratings.