Near Field Speckles

TL;DR

This thesis introduces three novel near-field speckle measurement techniques (ONFS, ENFS, SNFS) for analyzing light scattering from samples with non-uniform refractive indices, providing new ways to infer scattering intensities from speckle correlations.

Contribution

It presents the first mathematical derivation and experimental validation of three new near-field speckle techniques for light scattering analysis.

Findings

Correlation functions of speckle fields mirror sample correlations

Speckle field correlation functions can be used to calculate scattered intensity

Experimental results validate the theoretical formulas

Abstract

Elastic light scattering has been extensively used to study samples showing a non uniform refraction index on lengthscales from a fraction of a micrometer to a fraction of a millimeter. Typically, a wide laser beam is sent through the sample, and the light scattered at any angle is measured by a detector in the far field. In this Ph. D. thesis, I describe three new techniques, which allow to measure the scattering intensities, working in the near field: hOmoyne Near Field Speckles (ONFS), hEterodyne Near Field Speckles (ENFS) and Schlieren-like Near Field Speckles (SNFS). Basically, the experimental setup consists in a wide laser beam passing through the sample; a lens forms an image of a plane at a given distance from the cell on a CCD sensor. The image, in the near field, shows speckles, since it is formed by the stochastical interference of the light coming from a random sample. I show that, under suitable conditions, the correlation function of such a field closely mirrors the correlation function of the investigated sample; moreover, in general, from the correlation function of the speckle field it is possible to calculate the scattered intensity. I present for the first time a mathematical derivation of the working formulas for the three techniques, and experimental results.