Random photonic materials: Synthesis and characterization of light propagation
We study light propagation in strongly scattering, random photonic materials from material synthesis, sample fabrication, characterization of light propagation and theoretical calculation. Light propagation in random photonic materials is very important not only because the study can lead to better understanding of light propagation in ordered photonic materials (photonic crystal) ( i.e., the best filling fraction in photonic crystal, the coordination number to maximize the photonic band gap, etc.); and also because the light propagation in random materials can lead to fascinating physical problems (i.e., coherent backscattering, Anderson localization, and random laser etc.).
For the experiments, we synthesize the high index of refraction core-shell particles (ZnS-shell PS core, micron scale) with sonochemical methods. The smooth random films are fabricated by creating a concave meniscus from the colloidal solution. The structure (characterized by average coordination number Z) of high index particles is tuned by mixing the ZnS-PS with sacrificial PMMA spheres and followed by acetone wash. After the strongly scattering, random films are fabricated, the light propagation is characterized by measuring the coherent backscattering effect to obtain the transport mean free part of 1.06-micron wavelength light. We find a local minimum of l* (∼2.1μm) around Z∼4-5 and the scattering weakened with the increase of Z (Z>5). We show that the experimental results for porous random films disagree with the existing model for diffusive transport in random media. To explain our experimental discovery, we present a modified diffusion transport theory which incorporates the correlation of waves at strong scattering limit and Mie resonance regime to describe our experiments. The model should be useful to find the optimal conditions to enhance the scattering in random photonic materials.
Furthermore, we try to enhance the scattering in random photonic materials by changing the size of scatterers, index of refraction and incident laser wavelength not only in theoretical calculation according to our modified diffusion transport theory but also in the experiments. We synthesize high index of refraction material (SnS2, n∼3.0) whose index is characterized by single scattering method. We also synthesize metallic photonic materials (such as Gallium micro-spheres) whose index of refraction can vary dramatically in visible and near infrared regime. All these studies to enhance the scattering could lead to fascinating physical phenomena (i.e. Anderson localization, and random laser etc.).