Large nanoparticles (>40 nm) scatter light efficiently and can be visualized by traditional techniques like dark field scattering. However, as nanoparticles get smaller, their scattering signal is too weak to be detected directly and almost no light is detected. By interfering this weak scattering with light reflected at an interface, we can amplify the signal and image particles as small as 5 nm.

By scanning our excitation source, we build up iSCAT spectra, which are sensitive to the local environment of each nanoparticle. We observe transitions between positive (bright) and negative (dark) contrast as a function of wavelength.