Coherent extinction and self-homodyning by small particles.
Single-particle extinction using a coherent light source and where the Van Cittert-Zernike theorem applies to the forward-scattered light requires the angular summation of scattered fields instead of scattered intensities as done in the usual incoherent calculations. In spite of this approach, the resulting signals are calculated to be indistinguishable for single particles that are held stationary in the beam. When a flowing stream of particles moves perpendicularly through the beam one at a time, a modulated signal results which is described by an optical homodyning process and is called self-homodyning. The detection limit is a factor of 10 lower than that for incoherent extinction, allowing the detection of single submicrometer particles by simple extinction measurements. Theoretical calculations predicted the minimum detectable diameter for polystyrene latex (PSL) spheres to be 0.22 microm, and experimental measurements were able to detect PSL spheres of 0.31 microm.
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