Many authors investigated the onset of light localization in the regime of strong-disorder.
But how much strong?
Determining how much strong the disorder has to be to give rise to localized states (in three-dimensions) is an extremely difficult enterprise, as it depends on the specific kind of sample that is considered. One has to demonstrate that an optimal region for localization exists, once a parameter that quantifies the strength of the disorder has been identified. This is a very tricky problem, even more tricky if one wants to be far from a perturbative regime as the case of weakly disordered Photonic Crystals, considered here and here.
In a recent article (http://arxiv.org/abs/1003.2555), authored by Silvia Gentilini, Andrea Fratalocchi and Claudio Conti, a specific case has been considered by resorting to parallel large-scale Finite Difference Time-Domain simulations. A molecular dynamics code has been used to generate some disorder realizations of a colloidal solution composed by spherical particles. By varying the filling fraction of the resulting sample, the response of the material to an ultra-wide band (Frequency Comb) optical excitation has been spectrally analyzed.
It has been shown that the maximum spectral delay of the various frequencies in the transmitted spectrum displays a sharp peak in correspondence of a narrow region in the filling fraction for high-index contrast particles (figure below, delay expressed in picoseconds); this also being the first ab-initio numerical investigation of a Frequency Comb in a disordered material. Such a result shows that an optimal disorder for light localization can be indeed identified even for completely randomized samples.
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