Resonant scattering enhanced interferometric scattering microscopy.

Interferometric scattering (iSCAT) microscopy is a powerful tool for high-sensitive label-free imaging and sensing of nano-objects with high spatial-temporal resolution. The nano-objects imaged with the current iSCAT microscopy are usually non-resonant under laser light illumination and the iSCAT signal contrast is simply proportional to the volume and weight of the objects of interest. Here in this paper, we developed a novel strategy of resonant scattering enhanced iSCAT microscopy where the imaged nanoparticles are near resonant under laser light illumination, and we demonstrated it by using gold nanorods (NRs) with tunable longitudinal surface plasmon resonances. The obtained iSCAT signal contrast shows a dramatic variation in the narrow resonance wavelength range as small as 20 nm, and this is attributed to the strong wavelength dependence of the polarizability of gold NRs under optical resonance conditions. Different factors that have contributed to the iSCAT signal are theoretically analyzed and numerically simulated, providing the basic understanding about the effect of optical resonance on the iSCAT signal of nanoparticles. Our novel work provides a promising approach toward resonant sensing, imaging, and spectroscopy of nanoscopic objects.