Scanning surface plasmon imaging of nanoparticles

Confining surface plasmon resonance with a high-resolution heterodyne interferometric microscope, we demonstrate that subwavelength nanoparticles (10-200 nm in diameter) can be identified and sized in air and water media. Even though this microscope does not require any labeling of the nanoparticles, it offers the possibility to distinguish metal from polymer nanoparticles, by the identification of an angular transform of the backreflected field [V(Z)] versus the defocus Z. We propose a simple model that reproduces the optical response of the microscope in the presence of a nanoparticle perturbation. Both the shape of the microscope point spread function and the V(Z) curves compare well with the experimental data, confirming the possibility to discriminate metal from polymer nanoobjects with this microscope.