The Institute of Optics, University of Rochester
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Table of Contents
- Sample Preparation
- Light Microscopy
- Atomic Force Microscopy
- Secondary Electron Microscopy
- Stereo Pair
Butterfly wings, as one of the few photonic crystals in nature, have peculiar optical properties such as nearly total reflection at certain optical wavelength, angular dispersion of colors, etc., attributed to their special microscopic periodic structures. The goal of this project is to explore microscopic structures of butterfly wings with multiple microscopes and techniques, and to obtain data for potential analysis of their optical properties.
Since the sample has been in the jug for several years, it has already been air dried, and we did not need to dehydrate it. Butterfly wings have quite solid structures to support the shape, so air drying is applicable to them. In the sample preparation process, in order to prevent charging effect that is harmful to electron microscopy, we did sputter coating to the sample. We coated the butterfly wings for 180 seconds at an electric current of about 15 mA, which gave us a gold layer of about 18 nm.
Figs. 1 and 2 show light microscope images of butterfly wing scales. From these two images, we can see the contour and color of scales quite clearly. However, a main limitation of butterfly wing images taken by light microscope is depth of field, and only part of the scales are in focus. This effect is even worse for higher magnifications. Therefore, in order to get a better knowledge of the detailed structure of butterfly wings, we need to use microscopes with higher magnification and larger depth of field.
Fig.1 Light microscope image (10X)                      Fig.2 Light microscope image (20X)
Secondary Electron Microscopy
Scanning electron microscope is one option that has better properties in resolution, magnification and depth of field than light microscope. Figs 3-6 are all secondary electron images. Fig. 3 shows the scales. Shining holes where scales connect the substrate. From Fig. 4, we can see that in each scale, branches are pretty straight, and there are arrays connecting them, making a fine net. With the sample tiled, we got Figs. 5 and 6, from which we can tell the microscopic 3D structure of the scale. Branches and array connecting them are hundreds of nanometers deep, and there are also grids on the branch.
Fig.3 SE image of a few scales              Fig.4 SE image of microscopic structure of one scale
Fig.5 SE image of microscopic structures               Fig.6 SE image of nanoscale structures
with the sample tilted                                 with the sample tilted
In order to offer a better view of our sample, we used Photoshop to colorize our gray-level SE images. Scales are in orange, and substrates are in dark blue, and dusts are in dark red.
Fig.7 Colorized SE image of many scales               Fig.8 Colorizaed SE image of one scale
Atomic Force Microscopy
It is also helpful to obtain topography information of our sample. we utilized atomic force microscope (AFM) to do imaging as well. Since butterfly wing scales were not sticked tightly enough on the stage, we also sputtercoated the sample for AFM with gold with the same coating condition as for SEM. We scanned an area of 10 um x 10 um. Fig. 9 is the originally scanned two-dimensional image, and it can be converted into the topographical image Fig. 10. Limited by the geometry of the cantilever, we were only able to get the topography of the highest part the periodic structures, and there are artifacts such as "V" valleys, too.
Fig.9 Two-dimensional AFM image                            Fig.10 Topographical AFM image
We also used the stereo pair technique to show the depth of our image. First, we took a SE image. Then we tilted the stage by 3 degrees, and moved the stage so that the image area was in the raster again. Without changing the focal length, we adjusted the height of the stage so that the central part of the image was in focus again, and we took a second image. Finally, we colorized both images so that each can only be seen by one eye with a pair of red-blue 3D glasses on, and made a superposition with a proper shift between them. With 3D glasses, we could see the 3D effect in Fig. 11.
Fig.11 A steroe pair of SE images of scales
With light microscope, AFM and SEM, as well as image processing techniques such as colorization and stereo pair, we observed butterfly wings in various perspectives, and obtained a better understanding of their microscopic structures in different dimensions.
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