Optical Characterization of the Photonic Ball as a Structurally Colored Pigment

摘要： A photonic ball is a spherical colloidal crystal. Because it can exhibit vivid structural colors, many attempts have been made to apply it as a structurally colored pigment. However, the optical properties of the photonic ball are complicated because di?erent crystal planes can be involved in the coloration mechanism, depending on the size of the constituent colloidal particles. In this paper, we report a comparative study of photonic balls consisting of silica particles with sizes ranging from 220 to 500 nm. We ?rst analyze the re?ectance spectra acquired in a nearly backscattering geometry and con?rm that Bragg di?raction from di?erent crystal planes causes several spectral peaks. Second, the angular dependence of re?ection is experimentally characterized and theoretically analyzed with appropriate models. These analyses and a comparison with a planar colloidal crystal reveal that the spherical shape plays an essential role in the minor iridescence of photonic balls. We ?nally discuss a method to enhance color saturation by incorporating small light-absorbing particles. We also discuss the iridescence of the photonic ball under directional and ambient illumination conditions.