Refractive index and dispersion of butterfly chitin and bird keratin measured by polarizing interference microscopy

Leertouwer, Hein L.; Wilts, Bodo D.; Stavenga, Doekele G.

doi:10.1364/oe.19.024061

Cited by 195 publications

(187 citation statements)

References 24 publications

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“…The total reflection is remarkably low in the visible regime. In comparison with the bare glass, which is known to reflect about 8% of light under normal incidence (two surfaces 13 ), the glasswing reflection of 2% (two surfaces) in the visible regime is about four times lower while their refractive indices are quite close [14][15][16] . The low reflection is also observed for wavelengths in the ultraviolet to near infrared (NIR) regime.…”

Section: Resultsmentioning

confidence: 93%

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The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly

2015

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The glasswing butterfly (Greta oto) has, as its name suggests, transparent wings with remarkable low haze and reflectance over the whole visible spectral range even for large view angles of 80°. This omnidirectional anti-reflection behaviour is caused by small nanopillars covering the transparent regions of its wings. In difference to other anti-reflection coatings found in nature, these pillars are irregularly arranged and feature a random height and width distribution. Here we simulate the optical properties with the effective medium theory and transfer matrix method and show that the random height distribution of pillars significantly reduces the reflection not only for normal incidence but also for high view angles.

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Section: Resultsmentioning

confidence: 93%

“…For an easy comparison of different models and shapes, we use normalized scaling z Ã ¼ z h in the following. We assume a typical refractive index of chitin and air without absorption as n chitin ¼ 1.57 [14][15][16] and n air ¼ 1, respectively. The dash-dotted line in Fig.…”

Section: Dt Is the Complementary Error Function The Remaining Fractimentioning

confidence: 99%

The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly

2015

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confidence: 80%

“…While, unsurprisingly, higher refractive indices result in a stronger average reflectance, up to 84% for n = 2.8 (i.e., close to that of rutile titanium dioxide, a synthetic dielectric material commonly employed in photo nic applications [22] ), the broadband reflectivity rapidly decays for n < 1.6. Given the fact that the refractive index of chitin, varying from 1.6 to 1.55 (blue to red wavelengths, respectively), is among the highest found for purely organic biological materials, [23,24] this simulation illustrates yet another aspect of optical optimization of the network morphology.The careful balance of structural parameters described above begets the question whether the random network morphology exhibits hidden correlations that optimize the scattering of incident light. This question is motivated by the consideration that the optimization of the scattering strength requires an average distance between scatterers just above the wavelength of visible light (to avoid optical crowding) and the absence of periodic order (to avoid a photonic bandgap [25,26] ).…”

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confidence: 80%

Evolutionary‐Optimized Photonic Network Structure in White Beetle Wing Scales

et al. 2017

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Whiteness arises from the random scattering of incident light from disordered structures. [1] Opaque white materials have to contain a sufficiently large number of scatterers and therefore usually require thicker, material-rich nanostructures than structural color arising from the coherent interference of light. [2,3] In nature, bright white appearance arises from the dense arrays of pterin pigments in pierid butterflies, [4] guanine crystals in spiders, [5] or leucophore cells in the flexible skin of cuttlefish. [6] A striking example of such whiteness is found in the chitinous networks of white beetles, e.g., Lepidiota stigma and Cyphochilus sp. [7][8][9] Previous research investigating these beetle structures has shown that the chitinous network is one of the most strongly scattering materials in nature, and therefore the question arises whether this structure is evolutionary optimized for strong scattering while minimizing the Most studies of structural color in nature concern periodic arrays, which through the interference of light create color. The "color" white however relies on the multiple scattering of light within a randomly structured medium, which randomizes the direction and phase of incident light. Opaque white materials therefore must be much thicker than periodic structures. It is known that flying insects create "white" in extremely thin layers. This raises the question, whether evolution has optimized the wing scale morphology for white reflection at a minimum material use. This hypothesis is difficult to prove, since this requires the detailed knowledge of the scattering morphology combined with a suitable theoretical model. Here, a cryoptychographic X-ray tomography method is employed to obtain a full 3D structural dataset of the network morphology within a white beetle wing scale. By digitally manipulating this 3D representation, this study demonstrates that this morphology indeed provides the highest white retroreflection at the minimum use of material, and hence weight for the organism. Changing any of the network parameters (within the parameter space accessible by biological materials) either increases the weight, increases the thickness, or reduces reflectivity, providing clear evidence for the evolutionary optimization of this morphology. amount of employed material, thus reducing the weight of the organism. The brilliant white reflection from Cyphochilus beetles is assumed to be important for camouflage among white fungi and in a shady environment.In contrast to periodic photonic materials, for which the optical response is straightforward to calculate, the reflection of light from such disordered network morphologies requires a detailed knowledge of local geometry. [2,3,9,10] For these complex cases, the validity of the diffusion approximation is limited, since single scattering elements are difficult to be identified. [7] To fully understand the correlation between the structure and (optical) properties of complex materials, the detailed real-space structure in combination with a s...

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“…In the reflected spectrum, such interference appears as an oscillation and we refer to each reflectance peak as a spectral fringe. The wavelength‐dependent refractive index for chitin (Leertouwer, Wilts, & Stavenga, 2011) of n (λ) = 1.517 + 8,800 nm 2 /λ 2 was employed. The refractive index only varies 2% over the visible range, and the Brewster angle, θ Br = tan −1 ( n ), only changes less than 1° across the spectrum, though.…”

Section: Methods and Techniquesmentioning

confidence: 99%

Can the narrow red bands of dragonflies be used to perceive wing interference patterns?

Brydegaard

Jansson

Schulz

et al. 2018

Ecology and Evolution

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Despite numerous studies of selection on position and number of spectral vision bands, explanations to the function of narrow spectral bands are lacking. We investigate dragonflies (Odonata), which have the narrowest spectral bands reported, in order to investigate what features these narrow spectral bands may be used to perceive. We address whether it is likely that narrow red bands can be used to identify conspecifics by the optical signature from wing interference patterns (WIPs). We investigate the optical signatures of Odonata wings using hyperspectral imaging, laser profiling, ellipsometry, polarimetric modulation spectroscopy, and laser radar experiments. Based on results, we estimate the prospects for Odonata perception of WIPs to identify conspecifics in the spectral, spatial, intensity, polarization, angular, and temporal domains. We find six lines of evidence consistent with an ability to perceive WIPs. First, the wing membrane thickness of the studied Odonata is 2.3 μm, coinciding with the maximal thickness perceivable by the reported bandwidth. Second, flat wings imply that WIPs persist from whole wings, which can be seen at a distance. Third, WIPs constitute a major brightness in the visual environment only second after the solar disk. Fourth, WIPs exhibit high degree of polarization and polarization vision coincides with frontal narrow red bands in Odonata. Fifth, the angular light incidence on the Odonata composite eye provides all prerequisites for direct assessment of the refractive index which is associated with age. Sixth, WIPs from conspecifics in flight make a significant contribution even to the fundamental wingbeat frequency within the flicker fusion bandwidth of Odonata vision. We conclude that it is likely that WIPs can be perceived by the narrow red bands found in some Odonata species and propose future behavioral and electrophysiological tests of this hypothesis.

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Refractive index and dispersion of butterfly chitin and bird keratin measured by polarizing interference microscopy

Cited by 195 publications

References 24 publications

The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly

The role of random nanostructures for the omnidirectional anti-reflection properties of the glasswing butterfly

Evolutionary‐Optimized Photonic Network Structure in White Beetle Wing Scales

Can the narrow red bands of dragonflies be used to perceive wing interference patterns?

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