基于十字连通形环形谐振腔金属-介质-金属波导的折射率传感器和滤波器

Refractive index sensor and filter of metal-insulator-metal waveguide based on ring resonator embedded by cross structure

提出了由十字连通形环形谐振腔耦合两个金属-介质-金属(metal-insulator-metal, MIM)波导的结构,并用有限元法数值研究了表面等离极化激元在结构中的传输特性.通过对透射谱的研究,系统地分析了MIM结构的传感特性.结果表明,在透射光谱中有三个共振峰,即存在三种共振模式,其中透射峰与材料的折射率呈线性关系.通过对结构参数的优化,得到了折射率灵敏度(S)高达1500 nm/RIU的理论值,相应的传感分辨率为1.33×10^(-4)RIU.更重要的是,灵敏度不受结构参数变化的影响,这意味着传感器的灵敏度不受制造偏差的影响.此外,谐振波长与环形腔中心半径成线性关系,该器件在较大波长范围内实现可调谐带通滤波.透射强度随着波导与环形腔间距的增大而减小,透射带宽同时减小,因此,可以通过控制环形腔与波导的耦合距离来调谐透射强度及透射带宽.研究结果对高灵敏度纳米级折射率传感器和带通滤波器的设计以及在生物传感器方面的应用都具有一定的指导意义.

Continuous improvement in nanofabrication and nano-characterization capabilities have changed projections about the role that metals could play in developing the new optical devices. Surface plasmon polaritons are evanescent waves that propagate along a metal-dielectric interface. They can be laterally confined below the diffraction limit by using subwavelength metal structures, rendering them attractive to the development of miniaturized optical devices. A surface plasmon polariton refractive index sensor and filter which consist of two metal-insulator-metal（MIM） waveguides coupled to each other by a ring resonator embedded by cross structure are proposed. And the transmission characteristics of surface plasmon polaritons are studied in our proposed structure. The transmission properties of such a structure are simulated by the finite element method, and the eigenvalue wavelengths of the ring resonator are calculated theoretically.The sensing characteristics of such a structure are systematically analyzed by investigating the transmission spectrum.The results show that there are three resonance peaks in the transmission spectrum, that is, three resonance modes corresponding to the eigenvalue solutions of the first, second and third-order Bessel eigen-function equations, and each of which has a linear relationship with the refractive index of the material under sensing. Through the optimization of structural parameters, we achieve a theoretical value of the refractive index sensitivity（S） as high as 1500 nm/RIU, and the corresponding sensing resolution is 1.33×10^（-4） RIU. More importantly, it is sensitive to none of the parameters of our proposed structure, which means that the sensitivity of the sensor is immune to the fabrication deviation. In addition,by the resonant theory of ring resonator, we find a linear relationship between the resonance wavelength and the radius of ring resonator. So the resonance wavelength can be easily manipulated by adjusting the radius and refractive index.In addition, the positions of transmission peaks can be easily modulated by changing the radius of the ring, which can be used to design band-pass filter for a large wavelength range. Moreover, the transmission intensity and the transmission bandwidth decrease as spacing distance between the MIM waveguide and ring cavity increases. These results would be helpful in designing the refractive index sensor of high-sensitivity and band-pass filters, and have guiding significance for biological sensor applications.