Modeling the Reflectance Changes Induced by Vapor Condensation in Lycaenid Butterfly Wing Scales Colored by Photonic Nanoarchitectures.

Géza I Márk,Krisztián Kertész,Zsolt Bálint,Gábor Piszter,László P Biró

doi:10.3390/nano9050759

Géza I Márk, Krisztián Kertész + Show 3 more

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https://doi.org/10.3390/nano9050759

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Abstract

Gas/vapor sensors based on photonic band gap-type materials are attractive as they allow a quick optical readout. The photonic nanoarchitectures responsible for the coloration of the wing scales of many butterfly species possessing structural color exhibit chemical selectivity, i.e., give vapor-specific optical response signals. Modeling this complex physical-chemical process is very important to be able to exploit the possibilities of these photonic nanoarchitectures. We performed measurements of the ethanol vapor concentration-dependent reflectance spectra of the Albulina metallica butterfly, which exhibits structural color on both the dorsal (blue) and ventral (gold-green) wing sides. Using a numerical analysis of transmission electron microscopy (TEM) images, we revealed the details of the photonic nanoarchitecture inside the wing scales. On both sides, it is a 1D + 2D structure, a stack of layers, where the layers contain a quasi-ordered arrangement of air voids embedded in chitin. Next, we built a parametric simulation model that matched the measured spectra. The reflectance spectra were calculated by ab-initio methods by assuming variable amounts of vapor condensed to liquid in the air voids, as well as vapor concentration-dependent swelling of the chitin. From fitting the simulated results to the measured spectra, we found a similar swelling on both wing surfaces, but more liquid was found to concentrate in the smaller air voids for each vapor concentration value measured.

Highlights

The colors of butterfly wings [1,2,3,4,5,6] are generated by pigmentary and structural factors
Biological materials, always have a certain kind of disorder, and the disorder varies on a large scale among different butterfly species: from rigorously-ordered butterfly scales such as Parides sesostris [10,11] and Cyanophrys remus [12] to the lower end of the disorder scale as represented by seemingly amorphous structures such as those found in Albulina metallica [13]
No apparent order is seen in the arrangement of the small air voids on the top-view scanning electron microscopy (SEM) image (Figure 1a), but as shown in [28], the two-dimensional radial distribution function (2D RDF) of the center of the air voids showed a marked first neighbor shell on both the dorsal and ventral sides, confirming the presence of a radial short-range order and the lack of angular order

Summary

Introduction

The colors of butterfly wings [1,2,3,4,5,6] are generated by pigmentary (chemical) and structural (physical) factors. Both the pigments and nanostructures responsible for the color are located mostly in the wing scales. Optical properties of photonic crystal-type nanoarchitectures were first modeled by Yablonovitch [8] and John [9]. In its most general definition, a photonic crystal is a graded-refractive-index material, where the refractive index is a periodic function of the position in one-, two-, or three-dimensions, giving rise to a stop band [7]. Biological materials, always have a certain kind of disorder, and the disorder varies on a large scale among different butterfly species: from rigorously-ordered butterfly scales such as Parides sesostris [10,11] and Cyanophrys remus [12] to the lower end of the disorder scale as represented by seemingly amorphous structures such as those found in Albulina metallica [13]

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