title: Electron tomography, three-dimensional Fourier analysis and colour prediction of a three-dimensional amorphous biophotonic nanostructure.
abstract: Organismal colour can be created by selective absorption of light by pigments or light scattering by photonic nanostructures. Photonic nanostructures may vary in refractive index over one, two or three dimensions and may be periodic over large spatial scales or amorphous with short-range order. Theoretical optical analysis of three-dimensional amorphous nanostructures has been challenging because these structures are difficult to describe accurately from conventional two-dimensional electron microscopy alone. Intermediate voltage electron microscopy (IVEM) with tomographic reconstruction adds three-dimensional data by using a high-power electron beam to penetrate and image sections of material sufficiently thick to contain a significant portion of the structure. Here, we use IVEM tomography to characterize a non-iridescent, three-dimensional biophotonic nanostructure: the spongy medullary layer from eastern bluebird Sialia sialis feather barbs. Tomography and three-dimensional Fourier analysis reveal that it is an amorphous, interconnected bicontinuous matrix that is appropriately ordered at local spatial scales in all three dimensions to coherently scatter light. The predicted reflectance spectra from the three-dimensional Fourier analysis are more precise than those predicted by previous two-dimensional Fourier analysis of transmission electron microscopy sections. These results highlight the usefulness, and obstacles, of tomography in the description and analysis of three-dimensional photonic structures.
authors.affiliation: Department of Biology and Integrated Bioscience Program, University of Akron, Akron, OH 44325-3908, USA. mshawkey@nature.berkeley.edu
authors.forename: Matthew D
authors.initials: MD
authors.lastname: Shawkey
authors.forename: Vinodkumar
authors.initials: V
authors.lastname: Saranathan
authors.forename: Hildur
authors.initials: H
authors.lastname: Pálsdóttir
authors.forename: John
authors.initials: J
authors.lastname: Crum
authors.forename: Mark H
authors.initials: MH
authors.lastname: Ellisman
authors.forename: Manfred
authors.initials: M
authors.lastname: Auer
authors.forename: Richard O
authors.initials: RO
authors.lastname: Prum
mesh_terms: D000818:Animals; D055032:Electron Microscope Tomography; D005241:Feathers / Q000502:physiology*; D005583:Fourier Analysis; D008954:Models, Biological; D049329:Nanostructures; D055620:Optical Phenomena; D046109:Passeriformes / Q000502:physiology*