Optical Studies of Artificial Opals as 3D Photonic Crystals
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OPTICAL STUDIES OF ARTIFICIAL OPALS AS 3D PHOTONIC CRYSTALS D. Comoretto1, D. Cavallo1, G. Dellepiane1, R. Grassi2, F. Marabelli2, L.C. Andreani2, C.J. Brabec3,#, A. Andreev3 and A.A. Zakhidov4,* 1 INFM - Dipartimento di Chimica e Chimica Industriale, Università di Genova, Italy. 2 INFM - Dipartimento di Fisica “A. Volta”, Università di Pavia, Italy. 3 Christian Doppler Laboratory for Plastic Solar Cells, J. Kepler University Linz, Austria. 4 Honeywell International Inc., Research and Technology, Morristown, USA. # Present address SIEMENS AG, CTMM1- Innovative Electronics, D 91052 Erlangen, Germany. * Present address Department of Physics, University of Texas Dallas, Dallas, USA.
ABSTRACT Artificial opal films have been prepared by sedimentation of monodisperse silica spheres in water suspension. Atomic force microscope images show a triangular packing of the spheres at the surface of the films. The presence and the energy position of an optical pseudo gap in incidence-angle-dependent transmittance and reflectance spectra is observed and accounted for by theoretical calculations of the photonic band structure. These calculations indicate that the pseudo gap is due to the splitting of the photonic bands in the L point of the Brillouin zone. The spectroscopic data show additional loss structures due to both other features of the Brillouin zone and the diffraction of the light from the regular surface of the sample. The effect of the infiltration of opals with polydiacetylene solutions is also reported.
INTRODUCTION Photonic band gap crystals are widely studied for their optical properties that allow manipulation of the flux of the light thus making these materials good candidates for all optical signal processing [1]. When the photonic crystal exhibits the suitable structure, all the characteristics depend on the dielectric contrast between the component media, one of them being typically the void. The possibility to fill the void with photosensitive materials like conjugated polymers or molecules, is promising for photo-switching devices [2]. Photonic crystals for different applications can be grown with periodicity of the dielectric constant in one, two or three dimensions. 1D or 2D photonic structures in a wide spectral range can be grown both with the top-down or the bottom-up approaches. At present, the growth of 3D photonic crystals with photonic band gap in the visible or ultraviolet spectrum by the techniques typical of the top-down approach like electron beam or X-ray lithography is a technological challenge. The alternative bottom-up approach, based on the self-assembly processes of suitable building blocks, is also widely used. Nanospheres of silica or polystyrene can be assembled into ordered 3D structures. Energetic considerations show that the face centered cube (fcc) packing with the (111) direction perpendicular to the film surface is the most favorite one [3]. In this way, artificial opals, which mimic the charm of the well-known gem-stones and show interesting photonic properties can be produced [4
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