Numerical modelling of a nanoplasmonic biosensor based on a Mach-Zehnder interferometer

In the last few decades, optical biosensors based on surface plasmon resonance (SPR) have attracted increasing attention as a label-free alternative for the detection of small traces of biological and chemical markers, for application ranging from drug discovery and medical diagnosis to food quality and national defense. These approaches exploit the high sensitivity of surface plasmons polaritons (SPPs) to variations in the refractive index of the medium surrounding a thin metal film, which is caused by adsorption of the analyte molecules in the metal-dielectric interface. However, nowadays the plasmonic biosensor platforms with best performance require of complex optical configurations and bulky instrumentation, which difficult its miniaturization capability and portability, limiting its integration with other bioanalytical tools. In this work, we propose a novel design based on a Mach-Zehnder interferometer (MZI), consisting on a gold layer with a subwavelength aperture surrounded by grooves, and a detection system based on intensity interrogation. Our proposed architecture contemplates independent control of the reference and sensing arms in a planar disposition, which allows the biosensor to operate in the region of maximum sensitivity for low-analyte concentration and avoid the requirement of using complex multilayer fabrication techniques. Through numerical simulations using the FDTD-method, we found that our platform performed satisfactorily compared to previously reported designs. Moreover, its miniaturization potential, small footprint, and simple illumination scheme make it an ideal candidate for use in integrated sensing systems, which can be further enhanced by multiplexing.