Characterizing localized surface plasmon resonances using focused radially polarized beam

We demonstrate a scheme to characterize the localized surface plasmon resonances (LSPRs) of an individual metallic nanorod by employing a focused radially polarized beam (RPB) illumination under normal incidence. The focused RPB has a unique three-dimensional electric field polarization distribution in the focal plane, which can effectively and selectively excite the dipole and multipole plasmon resonances in a metallic nanorod by just moving the nanorod within the focal plane. This performance can be attributed to the mode matching between the excitation electric field of the incident RPB and the LSPRs in a metallic nanorod. Emphatically, in contrast to the commonly used oblique incidence illumination with the linearly polarized light, our proposed scheme is based on the normally incident light illumination and compatible with conventional optical microscopy, which is more scalable for spectroscopic characterization of individual nanostructures.