Abstract

We show that a small dielectric particle in a focused monochromatic light beam produces a scattered wave (Rayleigh scattering) in phase quadrature with the far field incident beam. We derive an expression for the resulting phase shift of this beam for a particle located at and near focus. Thus, a particle passing through the beam performs a point source convolution, tracing out the field amplitude (and phase) of the beam (e.g., showing typical Airy rings for an overfilled aperture). The forward scattered field due to the particle is detected using a bright field interferometer, which we describe, based on Nomarski optics. Since the sign of the scattered field is not lost, as in intensity measurements, particles in liquids may be distinguished from bubbles. We have verified the expected cubic dependence of the scattered field on particle radius by measuring the forward scattered field from single polystyrene spheres in water, in the 0.038-0.106-µm diam range. In addition, we have observed beam profiles in agreement with theoretical simulations.

© 1989 Optical Society of America