Atomically flat symmetric elliptical nanohole arrays in a gold film for ultrasensitive refractive index sensing

Past works on refractive index sensing using nanohole arrays in metal films typically achieved a resolution of around 10−4 to 10−5 refractive index units (RIU), up to 10−6 with complicated detection setups. This is an order of magnitude worse than commercial Kretschmann-based surface-plasmon resonance (SPR) sensors. Here, we demonstrate intensity-based bulk refractive index sensing in an aqueous environment with a resolution of 9.38 × 10−8 refractive index units (RIU), showing for the first time comparable performance for nanohole SPR with Kretschmann-based SPR. This is achieved by the combination of three advances in the materials properties: (a) template stripping to achieve ultra-flat Au surfaces of ∼0.2 nm roughness, (b) elliptical nanoholes to enhance transmission, and (c) a Cytop substrate to symmetrize the refractive index with the aqueous environment above the metal film. The simple optical microscope geometry and microfluidic integration used in this work is promising for multiplexed lab-on-chip analysis.