State Key Laboratory of Optoelectronic Materials and Technologies, School of Physics and Engineering, Sun Yat-sen University , Guangzhou 510275, China.
Nano Lett. 2014 Jul 9;14(7):3848-54. doi: 10.1021/nl501007d. Epub 2014 Jun 18.
We propose theoretically and demonstrate experimentally a dislocated double-layer metal grating structure, which operates as a unidirectional coupler capable of launching surface plasmon polaritons in a desired direction under normal illumination. The structure consists of a slanted dielectric grating sandwiched between two gold gratings. The upper gold grating has a nonzero lateral relative displacement with respect to the lower one. Numerical simulations show that a grating structure with 7 periods can convert 49% of normally incident light into surface plasmons with a contrast ratio of 78 between the powers of the surface plasmons launched in two opposite directions. We explain the unidirectional coupling phenomenon by the dislocation-induced interference of the diffracted waves from the upper and lower gold gratings. Furthermore, we developed a simple and cost-effective technique to fabricate the structure via tilted two-beam interference lithography and subsequent shadow deposition of gold. The experimental results demonstrate a coupling efficiency of 36% and a contrast ratio of 43. The relatively simple periodic nature of our structure lends itself to large-scale low-cost fabrication and simple theoretical analysis. Also, unlike the previous unidirectional couplers based on aperiodic structures, the design parameters of our unidirectional coupler can be determined analytically. Therefore, this structure can be an important component for surface-plasmon-based nanophotonic circuits by providing an efficient interface between free-space and surface plasmon waves.
我们从理论上提出并实验验证了一种错位双层金属光栅结构，该结构作为一种定向耦合器，在正常照明下能够在期望的方向上激发表面等离子体激元。该结构由夹在两个金光栅之间的倾斜介电光栅组成。上金光栅相对于下金光栅存在非零的横向相对位移。数值模拟表明，具有 7 个周期的光栅结构可以将 49%的垂直入射光转换为表面等离子体，在两个相反方向上激发的表面等离子体的功率对比度为 78。我们通过上、下金光栅的衍射波的错位干涉来解释这种定向耦合现象。此外，我们通过倾斜双光束干涉光刻和随后的金阴影沉积开发了一种简单且具有成本效益的制造技术。实验结果表明，该结构的耦合效率为 36%，对比度为 43。我们的结构具有相对简单的周期性，这使其易于进行大规模、低成本的制造和简单的理论分析。此外，与基于非周期性结构的先前的定向耦合器不同，我们的定向耦合器的设计参数可以进行解析确定。因此，通过在自由空间和表面等离子体波之间提供有效的接口，该结构可以成为基于表面等离子体的纳米光子电路的重要组成部分。
