Min Qiao, Chen Chengkun, Berini Pierre, Gordon Reuven
Dept. of Elec. and Comp. Eng., University of Victoria, 3800 Finnerty Rd, Victoria, BC, V8P 5C2, Canada.
Opt Express. 2010 Aug 30;18(18):19009-19. doi: 10.1364/OE.18.019009.
We show that long-range surface plasmons (LRSPs) are supported in a physically asymmetric thin film structure, consisting of a low refractive index medium on a metal slab, supported by a high refractive index dielectric layer (membrane) over air, as a suspended waveguide. For design purposes, an analytic formulation is derived in 1D yielding a transcendental equation that ensures symmetry of the transverse fields of the LRSP within the metal slab by constraining its thicknesses and that of the membrane. Results from the formulation are in quantitative agreement with transfer matrix calculations for a candidate slab waveguide consisting of an H(2)O-Au-SiO(2)-air structure. Biosensor-relevant figures of merit are compared for the asymmetric and symmetric structures, and it is found that the asymmetric structure actually improves performance, despite higher losses. The finite difference method is also used to analyse metal stripes providing 2D confinement on the structure, and additional constraints for non-radiative LRSP guiding thereon are discussed. These results are promising for sensors that operate with an aqueous solution that would otherwise require a low refractive index-matched substrate for the LRSP.
我们表明，长程表面等离激元（LRSPs）能在一种物理不对称的薄膜结构中存在，该结构由金属平板上的低折射率介质组成，并由空气上方的高折射率介电层（膜）支撑，作为一种悬浮波导。出于设计目的，在一维情况下推导了一个解析公式，得出一个超越方程，该方程通过限制金属平板及其膜的厚度来确保金属平板内LRSP横向场的对称性。该公式的结果与由H₂O - Au - SiO₂ - 空气结构组成的候选平板波导的传输矩阵计算结果在数量上一致。比较了不对称和对称结构与生物传感器相关的品质因数，发现尽管损耗较高，但不对称结构实际上提高了性能。有限差分法也被用于分析在结构上提供二维限制的金属条纹，并讨论了其上非辐射LRSP引导的附加约束。这些结果对于使用水溶液运行的传感器很有前景，否则这种传感器对于LRSP需要低折射率匹配的衬底。
