Examining the Scattering Elements of Natural and Engineered Photonic Crystals Using the Colorimetric Response of Morpho Menelaus Wings and 'Smart Slides' in the Presence of Fluids.
Mitropoulos, Alexander Nicholas.
2010
- Tropical Morpho butterflies are known for their bright blue iridescent colors, formed from an array of scales on the dorsal side of their wings. We present here an experimental study of the wing's spectral response by using multispectral, angle resolved analysis of the wing nanostructure in response to liquids of different indices of refraction and show sensitivity to changes in refractive index ... read moreof �n=0.01. We examine changes in scale number to find an optimal wing size that provides the narrowest bandwidth to be used for an optical sensor design. Individual wings of 4 mm x 4 mm samples were segmented using femtosecond laser machining to produce repeatable cuts to provide information regarding changes in scale number. These results underscore the opportunity to use these natural templates as colorimetric sensors for integration in optofluidic devices while illustrating the structural hierarchy present in the wing architecture in which the overall spectral signature appears as a collective effect caused by the disordered arrangement of individual scattering elements. We hypothesize an optimal structural hierarchy exists in the wing composed of highly sensitive individual scattering elements by monitoring the overall color change observed in a baseline image. Ordered scale portions provided a narrower spectral bandwidth with higher sensitivity, at the area of least disorder, to the same changes in index of refraction contrast. Results show there is a natural tunability of the colorimetric response depending on the number of scales used in each sample. This information introduces an additional variable which can be used to change the sensitivity of natural and engineered photonic crystals. Images taken at individual wavelengths were recorded using a multispectral imaging camera (CRI Nuance CCD camera and imaging software) to measure the peak wavelength response for individual samples after the addition of different oils. Further investigation with engineered two dimensional photonic crystals was used to monitor the sensitivity for colorimetric detection. The same methods performed on Morpho menelaus samples were repeated on nanoquilt 'smart slide' samples. Results show that both systems have similar characteristics with a linear peak shift as a function of added refractive index making both nanostructures useful for optofluidic sensor designs. However, 'smart slide' samples provide a highly ordered and repeatable colorimetric signature compared to Morpho samples, which, while presenting colorimetric complexity, do not present pattern recurrence between different scales. The 'deterministic disorder' of the engineered structures allows for the use of multiple scattering effects embedded in the nanostructure to track changes in structural frequencies instead of an overall compound colorimetric response.read less
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