Angle-independent structural coloured amorphous arrays

Abstract

We are able to observe a colour due to the interference of light from microstructures composed of different refractive index materials that is comparable to the visible wavelength of light; such a colour is called a structural colour. Because structural colour is fadeless and no energy is lost from the colour mechanism, structurally coloured materials are expected to be used for energy-saving reflective displays and sensors. Previously, however, the word ''iridescence'' rather than ''structural colour'' was used to describe the property of a surface that appears to change colour as the viewing angle or the angle of light illumination changes. Thus, people who are aware of the concept of interference colour have a strong impression that all structurally coloured materials change hue when viewed from different angles, as indicated by the term ''iridescence.'' In fact, most artificial structurally coloured materials that we and other groups have studied so far change their hue depending on the viewing and light illumination angles because these structural colours are derived from Bragg reflection. Such angle dependence presents a barrier for developing displays and sensors using structurally coloured materials. Therefore, my group has been working to develop angle-independent structural coloured materials. The latest most notable ones are amorphous array systems. In this review, I first introduce the microstructures and optical properties of low-angle-dependent structurally coloured amorphous arrays in biological systems, then describe the fabrication and the optical nature of the artificially prepared imitations of such biological systems, and finally, present the related theoretical studies.

Structurally Coloured Secondary Particles Composed of Black and White Colloidal Particles

“…Aggregations composed of fine submicron spherical colloidal particles (hereafter referred to as “colloidal particles”) can exhibit a decrease in the density of optical states (DOS) in the visible light range, depending on the states of aggregation and the contrast of the refractive index of the photonic structure12345. In many previous reports, the aggregations of colloidal particles were required to possess long-range order and periodicity, i.e.…”

Structural color coating films composed of an amorphous array of colloidal particles via electrophoretic deposition

“…To create amorphous structures from colloidal assemblies, simple evaporation, centrifugation, drop-casting, spin-coating and spray coating techniques have been commonly employed. [19][20][21][22][23][24][25][26][27][28][29][30] For example, we successfully prepared colorful coatings consisting of an amorphous array using mixtures of two different sizes of SiO 2 particles by the simple evaporation method. 20 In addition, we fabricated a colloidal amorphous array of SiO 2 particles by spray coating.…”

Optimization of sharp and viewing-angle-independent structural color

“…Structural randomness can decrease the angular dependence and is generally understood to be the reason for the non-iridescence of Morpho butterfly's blue color [28][29][30][31][32][33], but the randomization also broadens the bandwidth to about 100 nm or larger. Similarly, amorphous structures have weak angular dependence [34][35][36][37][38][39][40][41][42][43] but with spectra that are broad and with viewing-angledependent peak wavelengths when illuminated directionally [39]. Wavelength selectivity and angle insensitivity are hard to achieve simultaneously because interference effects couple the dependence on wavelength and the dependence on angle.…”