Diffraction minima resolve point scatterers at few hundredths of the wavelength

Abstract

Resolving two or more constantly scattering identical point sources using freely propagating waves is limited by diffraction. Here we show that, by illuminating with a diffraction minimum, a given number of point scatterers can be resolved at distances of small fractions of the wavelength. Specifically, we identify an 8 nm distance, which corresponds to 1/80 of the employed 640 nm wavelength, between two constantly emitting fluorescent molecules in the focal plane of an optical microscope. We also measure 22 nm side length for a quadratic array of four molecules. Moreover, we show that the measurement precision improves with decreasing distance and with increased scatterer density. This work opens up the prospect of resolving individual scatterers in clusters that are far smaller than the wavelength. The central maximum of a diffraction pattern is commonly used to resolve point-like scatterers. Now spatial resolution far below the diffraction limit is achieved by using the scattering minima.