Triple-parameter sensing in seawater using a dual-cavity tantalum-based plasmonic sensor

This work introduces a tantalum (Ta)-based metal-insulator-metal (MIM) plasmonic sensor employing an air-core bus waveguide coupled to two split resonators for multi-parameter seawater analysis. The sensing structure integrates a split octagonal cavity for dual-parameter measurement of salinity and temperature, and a split hexagonal cavity dedicated to detecting nanoplastic concentration through refractive index monitoring. Coupling between the air waveguide and each cavity is achieved using angle-optimized slit apertures, enabling enhanced mode excitation and minimized spectral overlap. Finite-element simulations reveal that the split octagonal resonator supports two well-separated resonant modes with maximum sensitivity of 5655 nm/RIU, while the split hexagonal cavity achieves a maximum sensitivity of 6948 nm/RIU. The resonance shifts between the two sensing channels remain spectrally isolated (Δλ _min  > 1400 nm), confirming low inter-cavity crosstalk. The use of Ta provides high corrosion resistance and structural stability in saline environments, ensuring reliable long-term operation. The proposed sensor achieves a compact footprint, high spectral selectivity, and simultaneous multi-parameter detection, demonstrating strong potential for real-time, in situ marine environmental monitoring and integrated photonic sensing platforms.