Degeneracy Breaking of Wood’s Anomaly for Enhanced Refractive Index Sensing

We
introduce an ultrasensitive detection technique for refractive
index (RI) sensing based on an array of nanometer scale slot-antennas
milled in a thin gold layer using a single lithographic step. Our
experimental figures of merit (FOMs) of 140–210 in the telecom
wavelength range approach the fundamental limit for standard propagating
SPR sensors (∼250).The underlying mechanism enabling this is
the combination of a narrowband resonance of the slot-antennas with
degeneracy breaking of Wood’s anomaly under slightly nonperpendicular
illumination. In addition, we explore the sensitivity of the device
to the depth of the analyte layer. This concept can be easily tuned
to any desired wavelength and RI range by modifying the slot dimensions
and the array spacing, thus rendering it highly useful for numerous
sensing applications.