Comparative Analysis of Metal-Oxide Thin Films on Planar Coverslip Waveguides for Low-Cost Lossy Mode Resonance Sensing

Introduction: Lossy mode resonance (LMR) sensors offer high sensitivity and support both TM and TE polarizations for label-free refractive index monitoring. However, most designs rely on complex optical fibers. This work proposes and numerically assesses a low-cost planar waveguide configuration using a standard glass coverslip coated with thin films as a compact sensor for aqueous and biological media.

Methods: The platform was modeled in FIMMWAVE/FIMMPROP using a soda-lime glass coverslip waveguide on a PMMA substrate. Five materials (SnO₂, ITO, In₂O₃, CuO, and Si) were evaluated, and the film thicknesses were optimized to locate the TM and TE resonances near 500 nm at a surrounding refractive index (SRI) of 1.333 RIU. The response was analyzed by varying the SRI from 1.330 to 1.380 RIU to calculate sensitivities for TM and TE polarizations.

Results: The configuration successfully supported LMR generation for all evaluated thin films with clear resonance shifts. For SnO₂, the sensor achieved a sensitivity of 1091 nm/RIU for the TM polarization and 1811 nm/RIU for the TE polarization. ITO exhibited sensitivities of 1046 nm/RIU (TM) and 1520 nm/RIU (TE), while In₂O₃ showed responses of 1033 nm/RIU (TM) and 1513 nm/RIU (TE). For CuO, the platform reached 668 nm/RIU for the TM polarization and 1537 nm/RIU for the TE polarization. Finally, Si demonstrated a TM sensitivity of 669 nm/RIU, while its TE resonance was omitted as it shifted into the deep infrared region. Across all materials, TE resonances yielded higher overall sensitivities due to enhanced evanescent field interactions at longer operating wavelengths.

Conclusions: Metal-oxide-coated coverslips are a promising low-cost platform for LMR refractive index sensing. Specifically, SnO₂ is the most suitable material due to its superior sensitivity and well-defined resonances. This planar configuration provides a simple, scalable route toward compact optical sensors for real-time chemical and biological monitoring in aqueous environments.