We present a set of experiments in which the backscattering spectra of 4 μm single TiO2 particles are probed with circularly polarized vortex beams. The experiment is carried out with a tunable laser at λ = 760 - 810nm. We observe that the use of vortex beams enables us to tailor the backscattering in different ways. Given a certain backscattering of a particle (induced by a Gaussian beam or a plane wave), we observe that vortex beams can tune it and sharpen it. Moreover, we also observe that the level of conservation of helicty can be increased thanks to the use of vortex beams. We explain the mechanisms that give rise to these effects using Mie Theory. Our method of backscattering control can be experimentally implemented in most of microscopy set-ups. In addition, if brought to its limits, the method can be used to excite single multipolar modes from spheres. We believe that our method could find application in the levitation of particles or the excitation of whispering gallery modes.
We will show how to extract information from the Mie coefficients to properly design dual systems combined with chiral
elements for having optically active structures. Such optically active elements will scatter the light omnidirectionally,
where the amount of rotation of light is fixed in any given direction independently of the incident polarization. The key
elements are the preservation of helicity by equaling the electric and magnetic responses from the material, and by the
proper manipulation of the angular momentum.
In dielectric structures such as photonic crystals that combine two types of materials it is likely that one may find a large degree of disorder. This is also true for nonlinear photonic crystals that combine two different orientations of the same material or a nonlinear with a linear material. Such a disorder may not always be detrimental for the propagation or generation of light. In the present work, we consider second harmonic generation in one-dimensional disordered nonlinear structures. When considering a random sequence of two different orientations of the same material, we show that second harmonic generation does not vanish but instead it exhibits a linear grow with respect to the number of domains considered. In structures that combine a nonlinear with a linear material, even when a large degree of disorder is introduced by allowing an extremely large dispersion in the size of the domains, the coherence of such second order nonlinear process is shown to survive.