  Polymeric materials are prized for their formability, low density, and
functional versatility. However, the refractive indices of common polymers fall
in a relatively narrow range between 1.4 and 1.6. Here, we demonstrate that
loading commercially-available polymers with large concentrations of a
plant-based pigment can effectively enhance their refractive index.For
polystyrene loaded with 67w/w\% $\beta$-carotene, we achieve a peak value of
2.2 near the absorption edge at $531~\mathrm{nm}$, while maintaining values
above 1.75 across longer wavelengths of the visible spectrum. Despite high
pigment loadings, this blend maintains the thermoforming ability of
polystyrene, and $\beta$-carotene remains molecularly dispersed. Similar
results are demonstrated for the plant-derived polymer ethyl cellulose. Since
the refractive index enhancement is intimately connected to the introduction of
strong absorption, it is best suited to applications where light travels short
distances through the material, such as reflectors and nanophotonic systems.We
experimentally demonstrate enhanced reflectance from films, as large as
seven-fold for ethyl cellulose at selected wavelengths. Theoretical
calculations that highlight that this simple strategy can significantly
increase light scattering by nanoparticles and enhance the performance of Bragg
reflectors.
