Numerical model to optimize the design of plasmonic optical fiber tips towards highly sensitive biosensing

Optical fibers have been explored as sensors for application in different fields, including diagnostics, food security, and environmental monitoring, due to their low size and weight, flexibility and electrical safety. Particularly, optical fiber biosensors based on surface plasmon resonance (SPR) are broadly reported given their rapid, label-free and highly sensitive sensing ability. Among them, plasmonic unclad tips are a cost-effective tool, in which light is reflected by the metal at the fiber end-face, eliciting SPR and doubling the optical path. Tips are recognized for their sensitivity, portability and mechanical strength, being more suitable for in-situ sensing compared to fragile or less compact sensors. In this work, a numerical model was developed to simulate the SPR curve of Au-coated tips (Au-tips) to readily predict their optimized design parameters and therefore enhance their sensitivity. The algorithm determined the reflected power using the three-layer Fresnel equation for p-polarization, with the SPR response being simulated for solutions with different refractive index (RI) surrounding the sensing area. The performance was then assessed by determination of the sensitivity. This way, a relationship was established between the sensor’s performance and its parameters, namely, the ratio between sensing region length and core diameter, numerical aperture and Au thickness. This simple method provided the idealized design parameters for an Au-tip sensor, which might have application for RI monitoring and as a highly sensitive biosensor.