Structural Color Circulation in a Bilayer Photonic Crystal by Increasing the Incident Angle

Abstract

Photonic crystals (PCs) have been widely applied in anti-counterfeiting field according to their easily tunable and angle-dependent structural colors. However, most studies are now focused on single-layer PCs assembled from monodisperse colloids spheres, which have only one bandgap. Here we prepared bilayer photonic crystals films by choosing 250 nm and 330 nm silica spheres as the bottom and top layer, respectively. The effect of the incident angle on the bandgap of PCs was investigated and the results showed that the bandgap of the bilayer PCs was incident angle dependent — the structure exhibited two strong bandgaps within small incident angles, while as increasing of the incident angle, both of the two bandgaps blue shifted, and more importantly, the bandgap of the bottom layer disappeared with further increasing incident angle. Furthermore, with the delicate design of the thickness of top layer, this bilayer structure selectively displayed the structural colors of the bottom layer, overlap colors of both the top and the bottom layer, and the color of only the top layer, respectively. By changing incident angle, the color circulation from green to magenta, orange, yellow, and green again was realized. The realization of the controllable color tunability further motived us towards the patterning of the bilayer PCs, which showed promising potential in the anti-counterfeiting field.