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Reflection and absorption of light in butterfly wing scales

Authors Zeng, Zheng
Issue Date 2008
Summary Coloration in butterfly scales is due to pigments or nanostructures. The iridescence of a scale arises from the selective reflection of light by the nanostructures in the scale. The most common nanostructure is the multilayers of alternating chitin and air. Dark color is the result of absorption by melanin pigment while white color arises from the reflection of all the incident light due to the absence of pigments. It is well known that butterflies warm up by basking in the sun. Solar heat is absorbed by the melanin pigment. We report here a direct SEM and FIB observation of terraced rather than uniform multilayer structure in the scales of Arhopala bazalus butterfly and Urania rhipheus moth. The number of layers in a scale increases from its base to apex. We have also observed ignition of the scales of Cyrestis thyodamas and Eurema hechbe butterflies after exposure to a conventional camera flash. Dark scales are ignited directly while white scales have to be coated with gold or silver before ignition is observed. Optical and SEM images show evidence of thermal damages. Flash ignition is an enhanced photothermal effect which involved the absorption of light, conversion to heat, confinement in nanostructures leading to huge increase in temperature. The absorption of light in scales is due to melanin pigment and localized surface plasmon resonance (LSPR) from the nanostructures of the Ag/Au films on butterfly scales. The view point that absorption of light is enhanced by LSPR is supported by surface enhanced Raman scattering (SERS) of Rhodamine 6G adsorbed on Ag/Au coated scales. The SERS enhancement factor obtained is greater than 105. Butterfly scales are basically macroscopic insulating structures with nanostructures. The enhanced photothermal effect in butterfly scales is therefore different from that of the single-walled carbon nanotubes and silicon nanowires.
Note Thesis (M.Phil.)--Hong Kong University of Science and Technology, 2008
Language English
Format Thesis
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