Dual-channel radially-polarized surface plasmon microscopy for simultaneous detection of fluorescence and linear scattering of nanospheres

We introduce and demonstrate a dual-channel radially-polarized surface plasmon microscopy (SPM) system with capability down to single nanoparticle detection. For nanospheres stained with fluorescent molecules, we are able to simultaneously collect both fluorescence and elastic scattering images. By using a radial polarizer, the entire incident beam is TM-polarized, which enables formation of a dark circular ring in the reflected image, thus providing higher sensitivity to refractive index changes. The fluorescence intensity is clearly enhanced by more than 50% under radial polarization as compared to a linear one. The complementary signals acquired from the two separated channels jointly lead to well-co-localized images in scanning mode. This technique is currently extended to study two photon fluorescence (TPF) signals from nanospheres, as well as second harmonic generation (SHG) signals from noncentrosymmetric nanocrystals. It also provides a way to compensate for the eventual blinking of the fluorescence, which does not affect the elastic scattering channel.