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Measurement of the size and refractive index of a small particle using the complex forward-scattered electromagnetic field.

We have previously shown that interferometry may be used to detect the phase shift of a focused beam caused by a small particle [Appl. Phys. Lett. 55, 215 (1989)]. This is a useful method for detecting small particles in liquids and with the ability to distinguish bubbles from particles. In that measurement, however, only the phase shift, given by the real part of the forward-scattered field, was detected. By measurement of both the real and imaginary parts of the forward-scattered field, additional information is obtained so that the refractive index and size of a nominally spherical particle may be determined. This measurement is analogous to ellipsometric measurements of thin films, where knowledge of both a phase and amplitude quantity allows one to determine a thin-film refractive index and thickness. Using an optical system based on Nomarski optics, we have shown that it is possible to distinguish four classes of particulate material in liquids within the noise constraints of our system. Thus low dielectrics (e.g., SiO(2)), moderate dielectrics (e.g., Si(3)N(4)), bubbles, and metals-absorbers (e.g., Al, C) can be differentiated as well as sized more accurately.