Multilayer metal-insulator-metal plasmonic vertical Mach-Zehnder sensor based on semi-circular slit-aperture nanostructures

Mach-Zehnder interferometer (MZI) sensors are widely used for gas and liquid detection, refractive index (RI) measurement, and real-time label-free detection in medical diagnostics, pharmaceutical researches, and implementation of food safety standards. Although, there is an increasing interest in miniaturizing these sensors for self-detection and out-of-lab detection, the performance of conventional metallic MZI sensors is still imperfect. In this paper, we propose a multilayer metal-insulator-metal plasmonic MZI to be utilized in multiplexed sensing. This simple interferometric sensor is composed of three semi-circular slit-aperture nanostructures connected together to form a curved MZ structure. In the semi-circular structure, interference take places between the surface plasmon polariton (SPP) waves that are propagating in the middle insulator (SiO₂) layer and SPPs at the interface between upper metal (Au) layer and air, where the analyte is placed. Finite difference time domain (FDTD) method is utilized to simulate the proposed structure. Simulations are performed for two scenarios: the case at which sample penetrates into the sensor structure and fills all the etched parts and the case at which sample only covers the surface of the sensor without any penetration. Our results show that a sensor with a footprint of 32.4 × 19 × 0.9 µm³ is capable of measuring the analyte refractive index with a sensitivity of 584 [nm/RIU ] for non-penetration case, and 960 [nm/RIU ] for the penetration case. Full wave at half maximum (FWHM) is calculated to be about 6.75 nm resulting in a figure of merit (FOM) values of 86 and 142 for non-penetration and penetration cases, respectively.