Tunable high-index photonic glasses

TL;DR

This paper introduces a tunable high-index photonic glass made of monodisperse TiO₂ spheres, demonstrating resonant scattering and tunable optical properties, with a validated theoretical model explaining their behavior.

Contribution

It presents a controlled high-index photonic glass system using TiO₂ colloids and a parameter-free model, advancing understanding of disordered photonic materials.

Findings

Resonant Mie scattering enhances turbidity.

Refractive index and particle size tuning control optical properties.

Model shows excellent agreement with experimental data.

Abstract

Materials with extreme photonic properties such as maximum diffuse reflectance, high albedo, or tunable band gaps are essential in many current and future photonic devices and coatings. While photonic crystals, periodic anisotropic structures, are well established, their disordered counterparts, photonic glasses (PGs), are less understood despite their most interesting isotropic photonic properties. Here, we introduce a controlled high index model PG system. It is made of monodisperse spherical TiO$_2$ colloids to exploit strongly resonant Mie scattering for optimal turbidity. We report spectrally resolved combined measurements of turbidity and light energy velocity from large monolithic crack-free samples. This material class reveals pronounced resonances enabled by the possibility to tune both the refractive index of the extremely low polydisperse constituents and their radius. All our results are rationalized by a model based on the energy coherent potential approximation, which is free of any fitting parameter. Surprisingly good quantitative agreement is found even at high index and elevated packing fraction. This class of PGs may be the key to optimized tunable photonic materials and also central to understand fundamental questions such as isotropic structural colors, random lasing or strong light localization in 3D.