Interferometric detection of forward scattered light from small particles

Abstract

The detection of subtenth micron paticles in processing fluids is a critical and growing need in the semiconductor industry. In this letter, 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, therefore causing a phase shift in this beam. Thus, the forward scattered field due to the particle may be detected using a bright field interferometer. This allows detection which is near Shott noise limited even for very small particles, and measures the sign of the scattered field as well, such that particles may be distinguished from bubbles. We describe an appropriate interferometer design based on Nomarski optics, which we have used to verify our calculation, measuring scattering in water from single polystyrene spheres as small as 0.038 μm in diameter.

Measuring the complex field scattered by single submicron particles

“…For example, a sharply focused laser beam was used in connection with optical traps for tracking nano beads, or measuring pico-Newton forces upon them. [10][11][12] Here we focus on the case of a larger particle, which is much more interesting [5][6][7][8] and in connection with our optical scheme represents the main focus of this paper. Let us bring one particle through the focal region, moving along the x axis as indicated by the arrows in Fig.…”

“…Several methods have previously been developed for measuring ϕ. Michelson, Mach-Zender and Smartt interferometers were used to get rid of the intense transmitted beam, as accurately described in literature. [5][6][7] A Nomarskii optical scheme was exploited for measuring the phase change of the whole transmitted wave. 5,6 More recently a simple method for measuring ϕ for a number of identical scatterers has been suggested, 8 based upon the statistical analysis of the speckle field formed by the superposition of the transmitted and the scattered fields just downstream of the sample.…”

“…[5][6][7] A Nomarskii optical scheme was exploited for measuring the phase change of the whole transmitted wave. 5,6 More recently a simple method for measuring ϕ for a number of identical scatterers has been suggested, 8 based upon the statistical analysis of the speckle field formed by the superposition of the transmitted and the scattered fields just downstream of the sample. The present method provides the real and imaginary parts of the complex field scattered by single particles through a relatively simple optical layout.…”

Scanning Head for the Apertureless near Field Optical Microscope

“…In such a case the interferometer used to detect the signal [12], which was already mentioned in the very first paper on ASNOM, is a vital part [1]. Without it, the dynamic range of the photodetector may be not sufficient [13] to recognize the signal component caused by the really near-field (non-radiative) part of the tip-sample electromagnetic interaction.…”

Heterodyne holographic microscopy of gold particles

“…To improve the detection sensitivity, interferometric approaches have been introduced [3,4,5,6,7]. Common-path interference, initially achieved with a Nomarski interferometer [8], translates phase variations into intensity variations, and enables the detection of phase perturbation provoked by spatial [9] or photothermal [10] modulation. Scanning heterodyne detection of the photothermal modulation [3] enables an unmatched combination of sensitivity and selectivity suitable to discriminate particles smaller than 5 nm from their background.…”