Abstract
The optical structures found in some weevils' scales are generally associated with photonic crystals. These structures are periodic along one, two or three dimensions. Suchperiodicities affect the propagation of electromagnetic waves inside the structured material through light interference. In addition to these optical effects, fluorescence emission can also occur in beetles' scales due to the presence of fluorophores which are naturally embedded within the structures' materials. This phenomenon is however underinvestigated. In this study, we analysed the fluorescence emission controlled and the coherent scattering within Eupholus linnei weevil's 3D photonic structures. This beetle is characterised by turquoise-striped black elytra. The turquoise colour of these stripes is due to scales exhibiting various photonic domains ranging from orange to blue under incident white light. Under ultraviolet light, the turquoise stripes emit light in the visible spectrum. Using electron microscopy, 3D photonic structures were observed in the weevil's scales. In addition to spectral measurements of the scales' reflectance and the analysis of these scales' colours in terms of chromaticity, the optical response of the observed nanostructures was modelled and predicted using the dominant reflected wavelength formula as well as optical simulation methods such as transfer-matrix simulation or the Rigorous Coupled-Wave Analysis (RCWA). Furthermore, the fluorescence properties of the scales were measured by spectrofluorimetry. The related decay time was evaluated to be longer at an emission wavelength within the photonic bandgap of the structures than outside this photonic bandgap. The quantum yield of the fluorescent scales was estimated to be rather low (about 21%), suggesting that fluorescence emission does not play a role in the visual appearance of the investigated weevil. The effect of the linear polarization was also studied and shows that the weevil is affected by the latter.
| Date of Award | 21 Jun 2019 |
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| Original language | French |
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| Supervisor | Olivier Deparis (Supervisor), Sebastien Mouchet (Co-Supervisor), Robert Sporken (Jury) & Alexandre Mayer (Jury) |