posted on 2022-12-28, 13:21authored byAnna B. Stephenson, Ming Xiao, Victoria Hwang, Liangliang Qu, Paul A. Odorisio, Michael Burke, Keith Task, Ted Deisenroth, Solomon Barkley, Rupa H. Darji, Vinothan N. Manoharan
Photonic balls are spheres tens of
micrometers in diameter containing
assemblies of nanoparticles or nanopores with a spacing comparable
to the wavelength of light. When these nanoscale features are disordered,
but still correlated, the photonic balls can show structural color
with low angle-dependence. Their colors, combined with the ability
to add them to a liquid formulation, make photonic balls a promising
new type of pigment particle for paints, coatings, and other applications.
However, it is challenging to predict the color of materials made
from photonic balls because the sphere geometry and multiple scattering
must be accounted for. To address these challenges, we develop a multiscale
modeling approach involving Monte Carlo simulations of multiple scattering
at two different scales: we simulate multiple scattering and absorption
within a photonic ball and then use the results to simulate multiple
scattering and absorption in a film of photonic balls. After validating
against experimental spectra, we use the model to show that films
of photonic balls scatter light in fundamentally different ways than
do homogeneous films of nanopores or nanoparticles, because of their
increased surface area and refraction at the interfaces of the balls.
Both effects tend to sharply reduce color saturation relative to a
homogeneous nanostructured film. We show that saturated colors can
be achieved by placing an absorber directly in the photonic balls
and mitigating surface roughness. With these design rules, we show
that photonic-ball films have an advantage over homogeneous nanostructured
films: their colors are even less dependent on the angle.