High-Sensitivity Bimodal Plasmo-Photonic Refractive Index Sensor

We present a new, ultracompact bimodal interferometric plasmonic sensor integrated on a SU-8 photonic waveguide platform. Two access SU-8 photonic waveguides are separated by an aluminum-based stripe, which resides on top of a thinner SU-8 waveguide layer. In this way, two metal/insulator interfaces are formed at the top and bottom metal surfaces, which are capable of supporting two respective surface plasmon polariton (SPP) modes that are subsequently interfering with the output photonic waveguide. The upper metal surface is exposed to the surrounding medium and serves as the sensing element, while the lower surface serves as the reference branch of the interferometer. After a thorough optimization process, the device was fabricated and experimentally characterized. A clear bimodal interferometric response was obtained when the upper metallic surface was exposed to air and water, with both cases revealing an excellent agreement between the simulated and experimental values for the free spectral range. The experimental bulk refractive index sensitivity of the bimodal interferometer was evaluated by using water solutions on top of the upper metallic surface, demonstrating experimental sensitivity values of 4386 nm/RIU that are in good agreement with the value of 5806 nm/RIU expected from the simulation. The proposed sensing device takes advantage of the polymeric material SU-8 for the photonic waveguide, reducing in this way fabrication time and overall fabrication cost.