IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 24, DECEMBER 15, 2006 2557 Structural Optimization of Silicon-On-Insulator Slot Waveguides Paul Müllner and Rainer Hainberger, Member, IEEE Abstract—In this letter, we theoretically investigate the poten- tial for further improvement of the slot waveguide structure in silicon-on-insulator at a wavelength of 1550 nm. Waveguide thick- ness and width, as well as slot thickness are optimized with respect to the optical power confined in the slot region by taking into ac- count the modal behavior of the structure. A single-mode criterion is defined as a function of the geometry parameters. Index Terms—Optical waveguides, silicon photonics, single- mode criterion. I. INTRODUCTION T HE progress in semiconductor fabrication technology has enabled the realization of photonic structures with deep-submicron feature size in ultrahigh-index contrast ma- terials, most prominently in silicon-on-insulator (SOI). Apart from photonic structures such as silicon wires [1]–[3] and photonic crystal slab waveguides[4], the so-called slot wave- guide [5]–[8] has attracted much attention. In a slot waveguide structure, the guided light is strongly confined in a narrow low-index gap between two high-index photonic wires. This enables the introduction of new photonic devices in which the characteristics of active optical materials can be efficiently exploited for modulation[9], switching, sensing, and other applications. Recently, the use of the slot waveguide structure for an electrically driven light-emitting device that is based on silicon has also been proposed[10]. In this work, we theoretically investigate the potential for fur- ther improvement of SOI slot waveguides. Waveguide thickness and width, as well as slot thickness are optimized with respect to the optical power confined in the slot region taking into account the modal behavior of the structure. II. SIMULATION MODEL We employ FEMLAB, a software tool for a full-vectorial two- dimensional finite-element method (FEM) eigenmode analysis, in order to study the characteristics of slot waveguides, as shown in Fig. 1 at a wavelength of 1550 nm. Triangular vector-elements were used, which eliminate spurious solutions and treat field- singularities at edges and corners correctly [11]. In order to de- termine the adequate mesh density required to achieve a suffi- cient precision, we first compared the calculated effective index of a one-dimensional slot waveguide with the analytic solution given in [6]. The mesh density was chosen such that the devia- tion between FEM results and analytic results was on the order of . Next, the mesh was refined for the two-dimensional Manuscript received June 31, 2006; revised September 21, 2006. The authors are with the Nano-System-Technology Division, ARC Seibers- dorf Research GmbH, 1220 Vienna, Austria (e-mail: paul.muellner@arcs.ac.at; rainer.hainberger@arcs.ac.at). Digital Object Identifier 10.1109/LPT.2006.886974 Fig. 1. Cross section of an SOI slot waveguide. Distance indicates the slot thickness and the waveguide thickness for fixed waveguide width . slot waveguide structure as depicted in Fig. 1 such that a com- parable accuracy for the effective index was obtained. The simu- lation domain was set to 4 m 4 m. Changing the boundary condition from a perfect electric conductor to a perfect mag- netic conductor had negligible influence on the simulation re- sults, thus indicating that the evanescent field has sufficiently decayed at the border of the simulation domain. III. RESULTS First, the dependence of the optical power confinement in the slot region of the fundamental quasi-TM mode, i.e., of that mode for which the electric field in the gap is oriented perpen- dicular to the slot interfaces, is studied as a function of the ge- ometry parameters. Previous studies [5]–[7] investigated the in- fluence of the slot thickness and the waveguide thickness for a fixed waveguide width of 300 nm. It was found that, for nm, up to one third of the guided optical power can be confined in the slot region if a waveguide thickness of about 180 nm is chosen. The confined power proved to depend critically on , whereas the influence of the slot thickness is much less. Over a wide range of the slot thicknesses between 50 nm and well beyond 140 nm, the optical power confined in the slot region remains nearly constant. For broader waveguides, the amount of optical power guided in the slot region can be further increased. For example, Fig. 2 plots the optical power confined in the slot region as a function of and for a waveguide width of nm. The max- imum power confinement of more than 45% is achieved for a waveguide thickness of approximately 157 nm, which is less than the value found for waveguides of width nm. We 1041-1135/$20.00 © 2006 IEEE