Characterizing the Backscattered Spectrum of Mie Spheres

This study describes both experimentally and theoretically an important hitherto undiscovered feature of the scattering of micron-sized spherical objects when illuminated with highly focused circularly polarized light. This is a regime of high experimental relevance which has not been described in full detail. The experiments are complemented with the analytical formulas explaining the field scattered directed toward the backward hemispace. In particular, it is proven that this field shows a very regular oscillatory dependency with the optical size. This phenomenon is typically hidden in the total scattered field, as the field is scattered much less toward the backward hemisphere than toward the forward one. These regular oscillations are measured experimentally. It is proven that, by analyzing them, it is possible to determine the index of refraction of isolated micron-sized particles, opening new paths for applications in sensing and metrology. A novel optical technique for the characterization of single spherical particles is presented. Relying on Mie Theory and its relation with the helicity of light, a regime is described in which the intensity of the back-scattering of micron-sized dielectric spheres shows an oscillatory pattern that is experimentally recreated and analyzed to determine the refractive index of isolated particles.image