Detailed Analysis of Peripheral Reflection from a Photonic Ball

Abstract

Colloidal crystals with spherical shapes are called photonic balls, and their optical properties have been extensively studied because they can be applied as structurally colored pigments or colorimetric sensors. When a colloidal crystal consists of small colloidal particles, e.g., 250 nm in diameter, the (111) crystal planes of the face‐centered cubic lattice cause Bragg reflection, resulting in a brilliant structural color. However, in colloidal crystals that consist of larger particles, e.g., 400 nm in diameter, different crystal planes may be involved in the coloration; the (220) planes cause ring‐like iridescent reflection in the peripheral part of the spherical crystal. Herein, a detailed optical analysis of this peripheral reflection is presented. The photonic ball consists of many small crystal domains, and the reflecting crystal planes are oriented in different directions from one domain to another. Thus, in some domains, significantly tilted planes and refraction at the spherical surface cause reflection in a highly oblique direction, such that the reflection path requires a 3D description. This study shows that such an oblique reflection significantly affects the appearance of the photonic ball.