Enhancing the Refractive Index of Polymers with a Plant-Based Pigment

Abstract

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% β-carotene, we achieve a peak value of 2.2 near the absorption edge at 531 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 β-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.