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

Photonic crystals (PCs) have been widely applied in the anticounterfeiting field according to their easily tunable and angle-dependent structural colors. However, most studies are now focused on single-layer PCs assembled from monodisperse colloidal spheres, which have only one bandgap. Here, we prepared bilayer photonic crystal films by choosing 250 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 the incident angle increases, both the bandgaps blue-shifted and more importantly, the bandgap of the bottom layer disappeared with a further increase in the incident angle. Furthermore, with the delicate design of the thickness of the 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 the 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 anticounterfeiting field.