Palestrante: Dr. Ernesto Jiménez Villar (Instituto de Ciência Molecular - Espanha)

Resumo: Anderson localization is one of the most interesting phenomena in solid-state physics. Particularly, localization of light is an open research frontier which, besides being a fundamental topic it also could present significant applications. Since Phillip Anderson proposed that scattering from disorder could bring transport to a complete halt, this phenomenon has greatly motivated scientists. In that seminal work, Anderson addressed the problem from a quantum vision, i.e taking into account the wave nature of the electron. Accounting that Anderson localization is a wave phenomenon (interferential), Sajeev john and Anderson himself extended this idea to optics. Optics seems an ideal framework to study localization and associated phenomena, due to non-interacting nature (seeming) of photons. In fact, various pioneering experiments that studied the transmission of electromagnetic waves through strongly disordered media have claimed the observation of Anderson localization of light. However, these works were questioned firstly by opponents and later refuted by their authors. The inelastic scattering processes (residual absorption or nonlinear phenomena) can lead to a decrease in the photon coherence length, hampering the interference effects (localization).

Our study in course, recently published at Nanoscale cover (Anderson localization of light in a colloidal suspension), ^1,2 reports several pieces of experimental evidence of localization of light in a colloidal suspension of core–shell nanoparticles (TiO₂@Silica). We demonstrate the crossover from a diffusive transport to a localization transition regime as nanoparticle concentration is increased, and that a striking phenomenon of enhanced absorption arises at localization transition from which an enhancement of effective refractive index was proposed (enhancement light-mater interaction). A decrease of optical conductance and an increase of absorption near the input border are reported when the incidence angle is increased. ³ The specular reflection, measured for the photons that enter the sample, is considerably lower than the effective internal reflection undergone by the coherently backscattered photons in the exact opposite direction, indicating a non-reciprocal propagation of light (parity-symmetry breaking). A theoretical simulation, performed through random-matrix theory, agrees satisfactorily with the experimental results, showing the generality of this approach to address transport phenomena. This strongly disordered medium in liquid suspension opens new avenues in the photonics field, ranging from the designing and manufacture of powerful sensing tools, novel photochemical reactors and other advanced photonic devices, ⁴ to investigations into fundamental topics, such as the light quantum nature and other phenomena involving photon interactions.

[1] Anderson Localization of light in a colloidal suspension (TiO₂@Silica) E.. Jimenez-Villar* et al. Nanoscale 2016, 8(21), 10938-10946.
[2] Core-shell TiO₂@Silica nanoparticles for light confinement. E. Jimenez-Villar* et al. Materials Today: Proceedings 2017, 4(11) part 2, 11570.
[3] Anderson localization of light: Strong dependence with incident angle” E. Jimenez-Villar* et al. arXiv:1705.09262 2017.
[4] Random lasing at localization transition in a colloidal suspension (TiO₂@Silica)” E. Jimenez-Villar* et al. ACS Omega 2017, 2, 2415–2421.