Surface vs. Bulk Sensitivity of Sensors Based on Rayleigh Anomalies in Metallic Nanogratings

This work provides numerical and experimental evidence that a significant difference exists between bulk and surface sensitivities in nanodevices based on Rayleigh anomalies (RAs) when employed for sensing the surrounding refractive index. In particular, recently proposed sensing schemes based on RAs in optical nanogratings are shown to be applicable only in the presence of bulk analytes. In the presence of thin overlays of analytes, instead, the (surface) sensitivity deteriorates up to two orders of magnitude by comparison with its bulk value, as well as with typical surface sensitivities exhibited by sensors based on surface plasmon resonances. This aspect is of fundamental importance as it severely limits the practical applicability of these devices to chemical and label-free biological sensing.