High-sensitivity polymer-based bimodal plasmonic refractive index sensors with polymer cladding

Plasmonic refractive index sensors are essential for detecting subtle variations in the ambient environment through surface plasmon interactions. Current efforts utilizing CMOS-compatible, plasmo-photonic Mach-Zehnder interferometers with active power balancing exhibit high sensitivities at the cost of fabrication and measurement complexity. Alternatively, passive bimodal plasmonic interferometers based on SU-8 waveguides offer cost-effectiveness and a smaller device footprint. However, their waveguides are exposed to ambient conditions and currently lack the necessary opto-mechanical isolation. In this work, we introduce the new SX AR LWL 2.0 polymer cladding material, which provides a high refractive index contrast to the SU-8 core, while ensuring compatibility with SU-8 waveguides. Our sensors consist of an aluminum plasmonic stripe and a bilayer SU-8 photonic waveguide core which is cladded with the SX AR LWL 2.0 polymer. They achieve a sensitivity of (6108 ± 13) nm/RIU (refractive index unit), where the error indicates the data fitting tolerance, and a limit of detection of 1.6 × 10^-7 RIU, in aqueous solutions, surpassing both traditional and polymer-based bimodal plasmo-photonic sensors. This new cladding material paves the way for more efficient and integrated bimodal plasmonic refractive index sensing platforms for a wide range of applications.