Abstract
Micrometer sized particles can be accurately characterized using holographic video microscopy and Lorenz-Mie fitting. In this work, we explore some of the limitations in holographic microscopy and introduce methods for increasing the accuracy of this technique with the use of multiple wavelengths of laser illumination. Large high index particle holograms have near degenerate solutions that can confuse standard fitting algorithms. Using a model based on diffraction from a phase disk, we explain the source of these degeneracies. We introduce multiple color holography as an effective approach to distinguish between degenerate solutions and provide improved accuracy for the holographic analysis of sub-visible colloidal particles.
Highlights
Holographic microscopy is a powerful tool for characterizing sub-visible particles in suspensions
We have found the existence of near-degenerate particle holograms generated by Lorenz-Mie theory for particles with significantly different refractive indexes
To better understand the mechanism behind these nearly degenerate solutions we developed a simple model of particle scattering where the particle is modeled as a cylinder or disk instead of a sphere
Summary
Holographic microscopy is a powerful tool for characterizing sub-visible particles in suspensions. For more complex particles, such as burning fuel particles, Wu et al have developed models based on Debye series and on generalized Lorenz-Mie theory and diffraction theory [17] These models have utility for accurately representing various particles, the interference structure of extinction coefficients can be explained by a geometric optics argument [9]. Higher index particles cause a larger phase delay, but when the phase delay gains 2π in phase the hologram returns to its initial form This approximate model provides a qualitative explanation for the degenerate solutions, but it is surprisingly accurate and captures much of the phenomena that we see in the exact Lorenz-Mie theory and in experiments. We show here the promise of the use of multiple wavelengths to expand the range of sizes and refractive indexes for holographic microscopy
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