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.

Shaping in the Third Direction; Fabrication of Hemispherical Micro-Concavity Array by Using Large Size Polystyrene Spheres as Template for Direct Self-Assembly of Small Size Silica Spheres

“…These diffraction rings in the peripheral part of the concavities seem to be a shape-dependent phenomenon which, although weak, could be constantly observed. Similar diffraction rings were reported in the case of photonic balls-colloidal crystals by R. Ohnuki et al, [ 62 ] when they were observed using objectives with different numerical apertures. They considered that the rings are caused by the tilted planes and refraction at the spherical surface which causes reflection in a highly oblique direction.…”

From Meso to Macro: Controlling Hierarchical Porosity in Supraparticle Powders

“…This morphology is also typically observed for SPs prepared under wellcontrolled drying conditions in emulsion-based systems. [7,63,64] The corresponding interstitial pores arising from such an arrangement are fully interconnected via necks formed from touching particles. We also quantify the pore size distribution within individual SPs from the tomographic reconstructions by inscribing a sphere of the largest possible diameter in each pore (maximum sphere inscription, MSI), [65] as exemplarily shown in Figure 2g, taken from Video S1 (Supporting Information).…”

Structural and Optical Characterization of Decahedral-Type Spherical Colloidal Clusters

“…However, photonic band gap calculations revealed that the band gap corresponding to Bragg diffraction from the ⟨100⟩ direction is closed, unlike the open band gap of the ⟨111⟩ direction. 9 Therefore, the tetrahedral unit in the center of Figure 4e appears dark. In the edge-ring pattern shown in Figure 4b, the green lines originate from the {111} plane of the outside-exposed faces of the five tetrahedra combined with refraction at the spherical surface.…”

“…This method, proposed by Velev et al, is widely used in fundamental and applied research. Some of the prepared clusters exhibit shell-like stacks of hexagonally arranged particle layers beneath the spherical surface. − These layers correspond to the (111) planes of the face-centered cubic (fcc) lattice, and this type of spherical cluster is called an onion-like structure because the stacked layers are similar to onion peels. Such onion-like spherical clusters, whose optical properties have been investigated comprehensively, are used in optical materials such as pigments, − colorimetric sensors, − and anticounterfeiting materials. − …”