Plasmonic interferometry for nanoscale refractometry enabled by quarter bull’s-eye gratings

We numerically study a double-slit interferometer that detects the modification of local phase and amplitude signals due to a refractive index contrast in a nanoplasmonic sensor after the plasmonic wave has been coupled to the far field. In particular, the sensor consists of two elongated zero-mode waveguides (ZMWs), which function to launch surface plasmons as well as transmit light directly into the far field. The surface plasmon waves are coupled to the far field by surface gratings, which cover one quarter of a bull’s-eye antenna, with different transmission directions. In the far field, the coupled plasmon waves interfere with the directly transmitted reference wave, which reveals a combination of the phase and amplitude modification by the asymmetry of refractive index in the lumen of the ZMW. We report a drastic increase in sensitivity over interferometers that only utilize the directly transmitted fraction.