Near Field Speckles The Optical Theorem Revisited

Abstract

The zero angle scattered wave plays an essential role in the celebrated Optical Theorem that relates the total extinction cross section (including both scattering and absorption) to the complex amplitude of the scattered wave. In an authoritative paper where the possibilities of an experimental verification of Optical Theorem were discussed, it was pointed out that a proof could hardly come from an optics experiment. One must agree to this statement if one receives the scattering intensity from an assembly of particles as a Bragg reflection from a three dimensional random grating. Accordingly, scattered waves at any angle then behave as a circular Gaussian process, and phases are equally distributed in the interval (-{\pi}, +{\pi}), therefore giving no cue on the behaviour of the wave scattered at exactly zero angle. So a proof of the Optical Theorem would require of telling apart an incoming wave from its weak replica, and gauge the phase in between. In my PhD thesis, near field scattering method is exploited for the determination of this elusive phase of the scattered wave. Having determined the phase of the scattered wave, the experimental verification of Optical Theorem can also be attempted. In addition, the method can generate dynamic scattering data as well; it provides an elegant and efficient way to filter out stray multiple scattering artefacts and thus the method can be used even when multiple scattering is rampant.