Alignment of crystal orientations of the multi-domain photonic crystals in Parides sesostris wing scales

Abstract

It is known that the wing scales of the emerald-patched cattleheart butterfly, Parides sesostris, contain gyroid-type photonic crystals, which produce a green structural colour. However, the photonic crystal is not a single crystal that spreads over the entire scale, but it is separated into many small domains with different crystal orientations. As a photonic crystal generally has band gaps at different frequencies depending on the direction of light propagation, it seems mysterious that the scale is observed to be uniformly green under an optical microscope despite the multi-domain structure. In this study, we have carefully investigated the structure of the wing scale and discovered that the crystal orientations of different domains are not perfectly random, but there is a preferred crystal orientation that is aligned along the surface normal of the scale. This finding suggests that there is an additional factor during the developmental process of the microstructure that regulates the crystal orientation.

Coexistence of both gyroid chiralities in individual butterfly wing scales of Callophrys rubi

“…The polycrystalline arrangement of the single-gyroid chitin structure shows a preferred alignment of certain crystallographic directions w.r.t the scale's outer surface normal, named crystallographic texture. Our findings of 12.5% and 32.5% of crystallites oriented along the <111> and <100> crystallographic directions, respectively, are in stark contrast to the observation in another gyroidforming butterfly, P. sesostris, where a strong prevalence of the <110> direction is observed (9). This difference in the degree of crystallographic texture between these two species is in accordance with the optical appearance of the wing scales, which show a rather uniform color in the case of P. sesostris and spatial variations of reflectivity for C. rubi.…”

“…It is an essential determinant of color uniformity, because the photonic properties of the single gyroid strongly depend on the crystal orientation (12,23). Yoshioka et al (9) observed a highly textured crystal orientation within scales of the butterfly Parides sesostris. Although the scales of P. sesostris exhibit a multicrystalline structure on the micrometer scale, they appear to be rather uniform in color, which is linked to the preferential orientation of the nonchiral <110> direction of the individual crystallites parallel to the surface normal of the scales.…”

“…Here, we investigate the chiral symmetry breaking of the singlegyroid structure, a complex network-like ordered nanostructure observed in the wing scales of various butterfly species, including Callophrys rubi (6)(7)(8)(9), and other arthropod species (10). The gyroid geometry, which serves as biophotonic crystals, has cubic symmetry (I4 1 32) and is characterized by the topologically particularly simple srs-net (the label for the chiral degree-three network modeled on SrSi2) (11).…”

It's Not a Bug, It's a Feature: Functional Materials in Insects

“…In insects, these are found in wing scales of nearly all families of butterflies, weevils, and beetles and are composed of cuticle sculpted into 3D minimal surfaces ( Figure 13D-F). [314][315][316][317][318][319][320] Light-matter interaction in these photonic crystals becomes highly dependent on the orientation of the photonic crystal, as well as the direction of incident light. Due to the low refractive-index contrast of cuticle and air (≈1.55), these photonic crystals cannot build a full photonic bandgap, but show a pronounced iridescence due to partial optical bandgaps that can be well explained by photonic bandgap modeling.…”

“…Each different morphology changes the way light interacts with the structure, as do local defects and disorder. Insect nanomorphologies range from ordered structures starting from thin films [309][310][311] and multilayer structures [277,282,304,312,313] to 3D photonic crystals to quasi-ordered and fully disordered structures, [314][315][316][317][318][319][320] each with different optical properties.…”