Amplitude and Phase Spectra of Light Scattered from a Single Nanoparticle

Abstract

Interferometric scattering microscopy (iSCAT), the imaging of interference between scattered light from a nanoparticle and reflected light from a substrate, provides a sensitivity sufficient for single biomolecule detection. The imaged interference, however, does not provide a separate amplitude and phase of the scattered light, making quantitative or spectroscopic measurement difficult. We report a method to fully recover the scattering amplitude and phase from iSCAT: vertical (z) scanning of the objective lens across the focus adds a Gouy phase between scattered light and reflected light, modulating image contrasts. The z- stack image analysis provides a highly accurate amplitude and phase of the scattered light. The method is validated by demonstrating that the recovered amplitude and phase spectra of a single gold nanoparticle reproduce well-known plasmon resonances. We also show that the phase spectra can be used to extract the damping parameter of the plasmon resonance and the coupling strength of the strongly coupled chromophore-nanoparticle assembly.