Many naturally occurring solids possess periodic structures that give rise to visible photonic crystal properties, [1] commonly termed structural colors. Some stunning examples are butterfly wings (one-dimensional, 1D), [2] abalone shells (1D), [3] sea-mouse spines (two-dimensional, 2D), [4] and natural opals (three-dimensional, 3D).[5] Exploitation of other periodic natural structures, is however limited by the inherently large size scale and the low dielectric contrast of the materials. Furthermore, these generally more complex geometries are a challenge to model correctly in order to obtain correct band diagrams. Here we report the development of a high fidelity cyclic size reduction and infiltration scheme, and apply it to a sea urchin exoskeleton to successfully fabricate a high dielectric contrast 3D photonic crystal exhibiting a stop band in the mid-IR range. The band structure of the exoskeleton is modeled using level set mathematics and agrees well with the experimental reflectivity exhibited by the 3D bicontinuous tellurium network of the replicated urchin.Periodic bicontinuous structures [6] are prevalent in systems such as surfactants, [7] block copolymers, [8] plant prolamellar bodies, [9] cell membranes, [10] and even in man-made playground architectures.[11] Naturally occurring bicontinuous structures are also found in the skeletons of sea urchins (termed stereom). [12,13] Stereoms are comprised of interpenetrating high magnesium calcite (incorporating up to 15 % MgCO 3 ) and soft tissue, with periodic spacings typically on the order of a few tens of micrometers. These labyrinthine composite structures are believed to have evolved to provide excellent nutrient access and to prevent crack propagation through the calcitic plates, although this latter notion has recently been challenged. [14] Some efforts to utilize these structures as templates for creating engineered material systems have previously been reported.[15±18] Among the various labyrinthine structures, the periodic structure termed ªrecti-linearº by marine biologists, [13] also commonly known as the ªPlumber's Nightmareº by the self-assembly community [19] and as ªSchwarz's P-surfaceº by mathematicians, [20] may exhibit unique optical characteristics. Rectilinear stereoms are found in many different sea urchin species, and we have used a stereom from the interambulacral plates of the Phyllacanthus species, having a unit cell with a lattice parameter of approximately 30 lm (see Fig. 1a).A perfectly periodic rectilinear stereom may be approximated using a level set equation for Schwarz's P-surface family [21] 10(cos x + cos y + cos z) ± 5.1(cos x cos y + cos y cos z + cos x cos z) = tThe structure has cubic Pm3 Å m symmetry; each node has six arms directed outwards along the three orthogonal directions. Varying the magnitude of t dilates or contracts the thickness of the arms defining the dividing surface and thus the network volume fraction of the bicontinuous structure. Calculations based on confocal microscopy and comparisons of scanning...

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A novel technique is described for making temperature measurements in a combusting flow. The technique, Thin-Filament Pyrometry, is based on the blackbody emission of a small ceramic filament (15 μm), which is introduced into the flow field under study. Because the emission along the entire length of the filament is recorded, the complete spatial temperature distribution is measured. The temporal response of the filament is ∼ 700 Hz under ambient conditions. To demonstrate the capabilities of the technique in a combusting flow, a H2-N2 jet diffusion flame was studied. The evolution of the large-scale buoyancy-driven structures at low Reynolds numbers was followed by this technique.

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