Photos of diatoms. (a) shows the photo of the whole shapes of the living cells of diatom surrounded silica glass (frustule). (b) shows the whole shape of the diatom, and (c) the cross section of a frustule of the diatom. While an inner part is a honeycomb structure, an outer layer has small holes on its surface. (d) shows the honeycomb structure from which outer layer is detached. (e) shows the surface of a outer layer. (f) shows the honeycomb structure. (a) is an optical microscopic image, (b)–(e) are SEM images, and (f) is a TEM image. The specimens for (a) and (c) were prepared by washing them in pure water, those for (d) and (e) by alkali treatment and one for (f) by hypochlorite and acid treatment as described, in text respectively. White scale bar, ; black scale bar, .
Apparent light absorption spectrum of the frustule measured by light irradiation through an optical fiber in air.
Calculational model of a diatom. (a) and (b) correspond to Figs. 1(c) and 1(d), respectively. Gray and blue regions are the silica and the background, respectively. A shell of the diatom is parallel to the 2D planes: , , , , and , , , where is the lattice constant of the honeycomb structure. The refractive index of silica is . The refractive index of a background is (air) or (water).
Photonic band structures of Fig. 3(a) and 3(b) show the photonic band structures in the backgrounds of air and water, respectively. An inset of a hexagon indicates the first Brillouin zone of triangular lattices. , , and points are symmetric points of the first Brillouin zone. While left and right vertical axes indicate the angular frequency (, where is the speed of light in vacuum and is the wavelength) and the wavelength, respectively, a horizontal axis indicates the 2D wave vector . Solid lines indicate the dispersion relations of frequencies and wave vectors. A gray region defined by is called the light cone in which light leaks into backgrounds.
Distributions of absolute values of magnetic fields of light confined inside shells. A dispersion relation with the lowest frequency is referred to as the band. While (a) and (b) show the distributions of the 1st (, ) and 12th (, ) bands at the point, respectively, (c) and (d) show the distributions of the 3rd (, ) and 18th (, ) bands at the point, respectively.
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