Disorder and broad-angle iridescence from Morpho-inspired structures.

Bokwang Song,Seok Chan Eom,Jung H Shin

doi:10.1364/oe.22.019386

Abstract

The ordered, lamellae-structured ridges on the wing scales of Morpho butterflies give rise to their striking blue iridescence by multilayer interference and grating diffraction. At the same time, the random offsets among the ridges broaden the directional multilayer reflection peaks and the grating diffraction peaks that the color appears the same at various viewing angles, contrary to the very definition of iridescence. While the overall process is well understood, there has been little investigation into confirming the roles of each factor due to the difficulty of controllably reproducing such complex structures. Here we use a combination of self-assembly, selective etching, and directional deposition to fabricate Morpho-inspired structure with controlled random offsets. We find that while random offsets are necessary, it alone is not sufficient to produce the broad-angle reflection of Morpho butterflies. We identify diffraction as a critical factor for the bright, anisotropic broadening of the reflection peak of Morpho butterflies to a solid angle of 0.23 sr, and suggest random macroscopic surface curvature as a practical alternative, with an isotropic broad reflection peak whose solid angle can reach 0.11 sr at an incident angle of 60°.

Highlights

Many insects possess intricate structures in the wings and scales that cause diffraction, interference, scattering, and many combinations thereof [1,2,3]
One particular phenomenon that has attracted a great deal of attention is iridescence, the property of certain surfaces to display brilliant structural colors that change color with the viewing angle [11,12,13,14,15]
This seeming paradox has attracted a great deal of attention, and the general principles have been well understood and investigated theoretically [19,20,21,22,23,24,25,26,27,28]: the blue color arises from interference within the multilayered ridges of chitin that cover the scales on the wings [20]; the dense packing of the narrow, discrete ridges lead to high reflectivity, and gives rise to additional effect of diffraction [21,22,23]; and the random offsets among the ridges removes the coherence, and broaden the sharp peaks associated with multilayer reflection and diffraction such that the color appears relatively stable across wide viewing angles [19, 24,25,26]

Summary

Introduction

Many insects possess intricate structures in the wings and scales that cause diffraction, interference, scattering, and many combinations thereof [1,2,3]. We have demonstrated that by spin-coating a close-packed monolayer of randomly sized silica microspheres, a substrate with random offsets can be generated and used to fabricate Morpho-inspired structural reflectors that show bright, angle-independent color [32] This approach differed from other, recent reports on successfully introducing randomness for broad-angle iridescence [33] in that the color was controlled by multilayer reflection as in actual Morpho butterflies, enabling accurate color tuning across a wide range, and that a continuous distribution of disorder, both vertical and horizontal, was generated spontaneously in a large area without any lithography or etching [34,35]. We find that random macroscopic surface curvature, which is often overlooked in theoretical analysis, can be quite effective in generating such bright, broad-angle reflection

Fabrication with disorder control

Reflectance measurement methods

Results and analysis

Conclusion