Why Blue Overwhelms Red

The brilliant blue and green hues seen on some birds, butterflies, and beetles arise from what’s known as structural color, which appears when nanostructures scatter light and certain wavelengths are amplified through interference. Materials that generate structural color, independent of the viewing angle, are called photonic glasses. But when it comes to yellow, orange, and red, photonic glasses underperform, and in nature, pigments such as melanins dominate instead. The reason that photonic glasses yield better color at one end of the visible spectrum is based on how nanoparticles inside the glasses scatter light, reports a team led by Vinothan N. Manoharan of Harvard University (Phys. Rev. E 2014, DOI: 10.1103/physreve.90.062302). Working with colloidal glasses, Manoharan’s team found that while the glasses as a whole might scatter in the red, individual particles within the glasses scatter more strongly in the blue and alter the overall hue. Previously, Manoharan’s group used a polymer-based core-shell approach to yield structural color across the full visible spectrum. The reds were weak (C&EN, April 28, page 28; Angew. Chem. Int. Ed. 2014, DOI: 10.1002/anie.201309306). The new work suggests that tuning the refractive index of the materials could yield more vivid hues.