Three-Dimensional Active Gratings for Light Emission Control
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scattering centres[3,4]. Two types of non-linearity are expected for such structures. These are the strong shift of the apparent Bragg spacing due to the non-linear dispersion [4] and the transformation of the emission spectrum of the medium incorporated in the grating voids due to the localisation of the light [2,4]. The photonic strength of these structures increases with increasing contrast of the refractive index between the dielectric balls nb and the liquid medium between the balls (nm) [4]. To achieve complete photonic band gaps the refractive index ratio (m=nb/nm) should be larger than 1.9. The main aim in this work is to examine the optical properties of the solid analogue of colloidal suspensions based on precious opal [5,6] surrounded by air. These solid gratings may be modified using additional chemical treatments with the aim of partially filling the structural voids with a "guest" medium possessing a higher refractive index. The obvious advantage of the opal over colloidal suspensions is its solid state, that would allow integration of such structures in optoelectronic devices. A recent report on synthesised opal comprising a fcc lattice of silica balls exhibited a "soft", or incomplete, photonic gap (n=1.45) [7]. We have deposited on the internal surface of the opal voids a thick rutile layer of higher refractive index (n=2.3) than silica. In first approximation the thickness of the layer providing the refractive indices contrast is not crucial and we proceed to analyse the characteristics of this complex material based on the same picture of the grating geometry as for the fcc package. The fcc lattice is not an ideal configuration for PBG action as it does not have a complete band gap in all directions [8]. However, the higher complexity of the unit cell of this structure (SiO 2 - TiO 2 - air) may lift the degeneracy of the photonic gap taking place in some directions for the opal fcc lattice [3]. Thus the prospect of demonstrating a partial PBG action has motivated this work. 289 Mat. Res. Soc. Symp. Proc. Vol. 406 © 1996 Materials Research Society
EXPERIMENTAL DETAILS Samples Samples are made from artificial opal with 200-300 nm silica ball diameters. In general the bare matrices can be divided in two groups. One group of samples (a) is a set of disordered porous media prepared from accidentally connected non-uniform balls with diameters in the same range as for ordered opals. The average distance between voids is 250 nm. The second group (b) is well ordered and partially ordered opal with regularly sized balls. In general the samples are infilled with amorphous silica using a silica-organic solution followed by drying process to glue balls with each other. Samples of regular sized balls having under gone such treatment are partially ordered. Unlike CVD deposition this forms islands of amorphous silica on the ball surface with size much less than the wavelength of light considered here. On the other hand, CVD deposition can achieve the control of the coating thickness with monolayer precision [9
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