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.
Highlights
The interaction of light with matter is of paramount importance for numerous technologies
We provided a model able to quantitatively describe light transport in densely packed photonic glasses (PGs) made of PS spheres [20]
Understanding the optical properties of white paints and appreciating the potential of PGs relies on materials suitable as model systems for quantitative comparison of a manageable theoretical model with experiments
Summary
The interaction of light with matter is of paramount importance for numerous technologies. It is of prime interest to understand light transport in high index disordered photonic materials quantitatively by using an appropriate transport theory which can be benchmarked by experiments Such analysis relies on model materials which have to fulfill a large number of requirements at once. We first describe the controlled preparation of crack-free monolithic high-index colloidal TiO2 PG samples at various refractive indices, particle sizes, and polydispersities. We determine their turbidities and energy velocities over the entire visible spectrum and compare them to empirically optimized commercial white paints. Very good overall agreement between measurements and our theoretical model is found
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