Low loss silica waveguides containing Si nanocrystals
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Low loss silica waveguides containing Si nanocrystals C. Garcia1, B. Garrido1, P. Pellegrino1, J.R. Morante1, M. Melchiorri2, N. Daldosso2, L. Pavesi2, E. Scheid3, G. Sarrabayrouse3 1 Electronics, University of Barcelona, Barcelona, Spain; 2 Physics, University of Trento, Trento, Italy; 3 LAAS - CNRS, Toulouse, France; ABSTRACT We study the optical and structural properties of rib-loaded waveguides working in the 600900 nm spectral range. A Si nanocrystal-rich SiOx with Si excess nominally ranging from 10 to 20% forms the active region of the waveguide. Starting materials were fused silica wafers and 2 µm-thick SiO2 thermally grown onto Si substrate. Si nanocrystals were precipitated by annealing at 1100ºC after quadruple Si ion implantation to high doses in a flat profile. The complete phase separation and formation of Si nanocrystals were monitored by means of optical tools, such as Raman, optical absorption and photoluminescence. Grain size distribution was obtained by electron microscopy. The actual Si excess content was obtained by X-ray photoelectron spectroscopy. The rib-loaded structure of the waveguide was fabricated by photolitographic and reactive ion etching processes, with patterned rib widths ranging from 1 to 8 µm. M-lines spectroscopy measurements provided a direct measurement of the refractive index and thickness of the active layers versus Si excess. When coupling a probe signal at 780 nm or 633 nm into the waveguide, an attenuation of at least 11 dB/cm was observed. These propagation losses have been attributed to Mie scattering, waveguide irregularities and direct absorption by the silicon nanocrystals. INTRODUCTION Si nanocrystals in amorphous matrices have been subject of intensive studies as a suitable optical active medium for CMOS-compatible photonic applications. The optical and structural properties of nanostructured silicon have been exhaustively investigated, and are relatively well understood [1, 2]. More recently, the important observation of optical gain from nanocrystalline Si has been reported [3, 4]. In this contribution we focus the attention in analysing the loss mechanisms in waveguide structures built with this material. EXPERIMENTAL DETAILS In order to introduce Si excess inside the oxide matrix, multiple Si+ ion implantations have been performed in both 2 µm thick SiO2 films thermally grown on Si and pure fused quartz wafers. Ion energies and relative doses of the multiple implantations were chosen to give rise to a “box-like” Si super-saturation down to a depth of about 0.4 µm. In such a way, the processing of the layers will allow to fabricate channel waveguides with a well defined homogeneous in-depth active media providing refractive index contrast with the claddings. The total ion doses were tailored to introduce a Si excess nominally ranging from 10% to 20% referred to the silica atomic concentration before implantation. Then the samples were annealed at 1100 ºC in N2 atmosphere for different durations, ranging from 1 min up to 16 h.
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