Modulating iridescence in structural colors through hierarchy, micro-geometry, but randomness (Conference Presentation)

Abstract

Iridescence has long been considered as an intrinsic special feature in structural colors. But we have also learned that iridescence in structural colors can be reduced by incorporating certain degrees of randomness, as demonstrated by the photonic glass and polycrystals. By studying structural colors in two different groups of spiders – the large, non-iridescent blue tarantulas and the tiny iridescent peacock spiders – we have found examples shown iridescence in structural colors can be both reduced and enhanced through the interaction of sub-micrometer photonic structural features, micrometer-scale geometries, and hierarchies without the needs of randomization. To fully understand how iridescence is affected by structures of specialized hairs and scales in these spiders, we use an interdisciplinary biomimetic approach and apply a myriad of characterization, simulation, and prototyping techniques during our investigations, including but not limited to, spectrophotometry, electron microscopy, imaging scatterometry, hyperspectral imaging, finite element analysis, and two-photon nanolithography. We find that blue structurally colored hair from some tarantulas shows multilayer structures with hierarchical cylindrical groove-like configurations along their length, and determine that this hierarchical geometry with its high degree of rotational symmetry (flower-like shape) almost entirely eliminates the iridescence produced by the multilayer structure, producing highly consistent blue color from all viewing angles. On the other hand, the rainbow iridescent scales from some peacock spiders combine nanoscale 2D diffraction grating structures with microscale 3D convex curvature, enabling the scales to separate different wavelengths of light at a higher resolving power than the conventional, flat diffraction grating.