Grating Diffraction or Bragg Diffraction? Coloration Mechanisms of the Photonic Ball

Abstract

Abstract The coloration mechanism of the photonic ball is investigated, which is a spherical aggregation of submicrometer‐sized colloidal particles. An interesting optical property is reported for photonic balls with colloidal particles larger than 400 nm: a ring‐like iridescent reflection appears on the peripheral part of the ball when observed under an optical microscope. Previous studies considered grating diffraction or Bragg diffraction separately to explain this iridescence. Here, it is shown from detailed structural and optical investigations that both diffraction mechanisms are important. In particular, Bragg diffraction explains the presence of a specific color for the photonic ball when observed with the naked eye in contrast to the iridescence seen under the microscope. The essential idea of the optical analysis is to combine the Bragg diffraction that occurs in the 3D periodic structure with the grating diffraction that originates from the surface with a 2D periodicity. The macroscopic color is almost independent of the angle of observation because of this unique reflection mechanism; hence, the photonic ball can be a suitable candidate for the structurally colored pigment.