  Plasmonic nanostructures with large local field enhancement have been
extensively investigated for sensing applications. However, the quality factor
and thus the sensing figure of merit are limited due to relatively high ohmic
loss. Here we propose a novel plasmonic sensor with ultrahigh figure of merit
based on super-narrow Rayleigh anomaly (RA) in a mirror-backed dielectric
metasurface. Simulation results show that the RA in such a metasurface can have
a super-high quality factor of 16000 in the visible regime, which is an order
of magnitude larger than the highest value of reported plasmonic
nanostructures. We attribute this striking performance to the enhanced electric
fields far away from the metal film. The super-high quality factor and the
greatly enhanced field confined to the superstrate region make the
mirror-backed dielectric metasurface an ideal platform for sensing. We show
that the figure of merit of this RA-based metasurface sensor can be as high as
15930/RIU. Additionally, we reveal that RA-based plasmonic sensors share some
typical characteristics, providing guidance for the structure design. We expect
this work advance the development of high-performance plasmonic metasurface
sensors.
