基于金属-电介质-金属(MDM)波导侧向耦合倾斜半环形谐振腔结构, 提出了一种结构紧凑且具有高灵敏度响应的折射率传感器模型。 引入与水平方向成70°角倾斜分布的半环腔, 可以有效地打破波导耦合谐振腔结构的对称型, 激发出更多的谐振模式。 采用有限元法计算出了含金属挡板MDM波导结构耦合有效半径为185 nm的倾斜半环腔结构的透射谱线, 三个具有非对称Fano线型的共振透射峰分别出现在594, 868及1 734 nm的波长位置。 透射峰值对应的模场分布揭示了三个透射峰分别对应于半环腔中的三阶、 二阶和一阶谐振模式, 记为FR3, FR2及FR1。 基于半环谐振腔内的1～3阶窄带谐振模与波导内金属挡板产生的宽带反射模之间的耦合干涉效应解释了透射谱线中的非对称Fano透射峰的形成机理。 同时基于半环腔中的类FP谐振条件推出了折射率传感灵敏度的近似解析计算公式, 揭示了折射率传感灵敏度近似与腔长L呈正比关系, 与谐振阶次m成反比关系的规律。 通过改变介质层中的折射率参数, 得到了透射峰FR3, FR2及FR1的折射率传感响应灵敏度分别为550, 840及1 724 nm·RIU-1。 最后在保持曲率半径不变的前提下, 延伸半环腔的弧长构建了开口角为π/2的开口环形腔结构, 实现了腔长L的1.5倍增长。 数值计算表明开口环形腔耦合MDM波导结构中的三重透射峰折射率传感灵敏度进一步提升到821, 1 250及2 517 nm·RIU-1, 相对半环腔结构均近似实现了1.5倍的提升。 数值结果进一步验证了近似解析公式的有效性。 研究结果为实现结构紧凑的高灵敏度折射率传感器设计提供了理论基础。

A compact plasmonic refractive index sensor with ultra-high sensitivity has been proposed based on a metal-dielectric-metal waveguide coupled with atilted semi-ring cavity. A semi-ring cavity with a tilted angle of 70° to the horizontal direction is introduced above the MDM wave guideso that the symmetry characteristics of the resonator have been broken and more resonance modes can be generated. The transmission spectrum of a baffle-contained MDM waveguide coupled with a tilted semi-ring with a radius of 185 nm has been calculated numerically through the finite element method. Three transmission peaks with asymmetric Fano line-shape appear at the wavelengths of 594, 868 and 1 734 nm, respectively. The magnetic field distributions at these three wavelengths demonstrate that the third, second and first order resonance modes occurred in the semi-ring cavity lead to the three peaks in the transmission spectrum, which FR3, FR2 and FR1, respectively denote. Based on the coupling interference effect between the first three order narrow resonant modes and wide band reflected propagation modes in the MDM waveguide generated by the metal baffle, the formation mechanisms of triple Fano resonances leading to the three asymmetric transmission peaks have been clarified in detail. Meanwhile, the dependence of peak wavelengths on the refractive index of the dielectric materials has been calculated numerically, and the sensitivity associated with FR3, FR2, and FR1 are thus obtained as 550, 840 and 1 724 nm·RIU-1, respectively. In addition, an approximate analytical formula of the sensing sensitivity has been derived from the resonant condition of the semi-ring cavity, which displayed the linear dependence of the refractive index sensing sensitivity on the cavity length L and the reciprocal of the resonant order m. Obviously, the calculated three sensitivity values of the semi-ring cavity approximately satisfy this relationship. Finally, a split-ring with a split angle of π/2 has been obtained by extending the arc length of the semi-ring cavity without altering the radius of curvature so that the cavity length L has been lengthened 1.5-fold by increasing the center angle from π to 3π/2. Numerical results demonstrated that the sensing sensitivity of the three Fano peaks is increased to 821, 1 250 and 2 517 nm·RIU-1, respectively, which are 1.5 times the original values achieved in the semi-ring cavity scheme. Numerical results further verify the effectiveness of the approximate analytical formula, which provides a theoretical guideline for the design of compact high-sensitivity refractive index sensor.