Role of Anisotropy and Refractive Index in Scattering and Whiteness Optimization

Gianni Jacucci, Jacopo Bertolotti, Silvia Vignolini
2019 Advanced Optical Materials  
structure factor) and their specific characteristics (form factor and refractive index). [1] [2] [3] [4] Therefore, most efforts to control light propagation in random systems have been focused upon the optimization of isotropic spatial correlations in highrefractive index systems. These studies had a remarkable impact in many fundamental [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] and applied phenomena. [17] [18] [19] [20] [21] However, the role of anisotropy in multiple scattering
more » ... ystems has been overlooked. In fact, despite the well-known analytical solution for the single scattering, [4, 22] the response of an ensemble of randomly arranged anisotropic particles has not been theoretically investigated. The same is valid for the presence of anisotropic structural correlations. Similarly, recent experimental works focused on the fabrication and optical characterization of anisotropic, disordered materials but without proving the role of anisotropy in scattering optimization. [23] [24] [25] [26] [27] [28] [29] [30] Here, we study the effect of structural and single-particle anisotropy on the opacity of a material and identify the criteria to improve scattering over a large parameter space, including filling fraction and refractive index. Our work proves that ensembles of uncorrelated, anisotropic particles outperform their isotropic counterpart both for low-and high-refractive indices. In addition, anisotropic, low-refractive index systems not only require a lower amount of material to maximize scattering than isotropic media, but they also exhibit a whiteness comparable to those of high-refractive index materials. Our results showcase how to exploit natural resources (e.g., biopolymers) to replace commercially available white enhancers, made from inorganic high-refractive materials (e.g., TiO 2 ), which have recently raised safety concerns. [31, 32] Moreover, our work unveils why anisotropy is an evolutionary-chosen mechanism to optimize scattering in biological systems. Results and Discussion To establish a comparison between different disordered materials, we numerically investigated the optical properties of 2D media (see Sections S1 and S2 in the Supporting Information). Exploiting a 2D numerical approach allows to independently vary the structural and form factors of a system, and therefore to disentangle their role in scattering optimization, while avoiding computational burden. In particular, we systematically The ability to manipulate light-matter interaction to tailor the scattering properties of materials is crucial to many aspects of everyday life, from paints to lighting, and to many fundamental concepts in disordered photonics. Light transport and scattering in a granular disordered medium are dictated by the spatial distribution (structure factor) and the scattering properties (form factor and refractive index) of its building blocks. As yet, however, the importance of anisotropy in such systems has not been considered. Here, a systematic numerical survey that disentangles and quantifies the role of different kinds and degrees of anisotropy in scattering optimization is reported. It is shown that ensembles of uncorrelated, anisotropic particles with nematic ordering enables to increase by 20% the reflectance of low-refractive index media (n = 1.55), using only three-quarters of material compared to their isotropic counterpart. Additionally, these systems exhibit a whiteness comparable to conventionally used high-refractive index media, e.g., TiO 2 (n = 2.60). Therefore, the findings not only provide an understanding of the role of anisotropy in scattering optimization, but they also showcase a novel strategy to replace inorganic white enhancers with sustainable and biocompatible products made of biopolymers.
doi:10.1002/adom.201900980 fatcat:qokrrphdtzaavfita3lqofstsa