Local Refractive Index Dependence of Plasmon Resonance Spectra from Individual Nanoparticles

We present an experimental optical darkfield microscope study of the dependence of the plasmon resonance spectrum of individual silver nanoparticles on the local index of refraction. We systematically characterize the position of the resonance peaks associated with the same set of individual silver nanoparticles embedded sequentially in index oils with increasing refractive index. This technique effectively allows the local refractive index to be stepped in increments of 0.04. As the local index is increased, the spectrum from each of the nanoparticles generally undergoes a very regular and reproducible red shift; however, we find that the amount of red shift per index increase varies depending on the shape of the nanoparticle and the mode of excitation. In particular, we find that the spectral peak that occurs in triangular nanoparticles exhibits a noticeably larger red shift than that associated with the dipole mode corresponding to spherical nanoparticles. Our results are consistent with experiments performed on ensembles of similar nanoparticles and suggest that individual nanoparticles may be utilized in biosensing applications where currently ensembles are being investigated.