Plasmonic Spectroscopy: The Electromagnetic Field Strength and its Distribution Determine the Sensitivity Factor of Face-to-Face Ag Nanocube Dimers in Solution and on a Substrate

Using the DDA method, the spectral sensitivity of a 42 nm Ag nanocube dimer oriented face-to-face in solution and placed on a substrate was investigated. The sensitivity factor was calculated as a function of both the plasmonic band that was analyzed and the refractive index of the substrate. We found that the maximum value of the electromagnetic field intensity of a given plasmonic band is not directly proportional to its extinction intensity. Also, the sensitivity factor of a plasmonic band does not depend on its extinction intensity, but rather, it is a function of the maximum value of the plasmonic electromagnetic field intensity distribution. The presence of a high refractive index substrate drastically affects the extinction intensity of a given band and selectively enhances specific plasmonic bands on the basis of their electromagnetic field distribution. The distribution of the electromagnetic field intensity on the surface of the Ag nanocube dimer is also affected by the presence of a substrate with a high refractive index. As the refractive index of the substrate increases, the substrate serves to reduce the maximum value of the plasmonic field intensity. Consequently, and as is known, the sensitivity factor decrease as the refractive index of the substrate increases. This suggests that when designing a good sensor, it is imperative to use a substrate with a dielectric function smaller than that of the solutions being detected.