Silicon-on-Insulator Slot Waveguides: Theory and Applications in Electro-Optics and Optical Sensing [chapter]

Patrick Steglich
2018 Emerging Waveguide Technology  
This chapter deals with the basic concept of silicon-on-insulator (SOI) slot waveguides, including slot waveguide theory, fabrication steps, and applications. First, in the theory section, a modal field expression and the characteristic equation is derived, which is also valid for higher-order modes. SOI slot waveguide structures are simulated and characteristic values like the effective refractive indices and the field confinement factors are determined. The fabrication section describes
more » ... ion describes typical SOI fabrication steps and the limits of current fabrication techniques. Additionally, developments regarding loss reduction in SOI slot waveguides are given from the fabrication point of view. This is followed by the theory and practice of slot waveguide based electro-optical modulators. Here, the SOI slot waveguide is embedded in an organic nonlinear optical material in order to achieve record-low voltage-length products. In the field of optical sensors, it is shown that slot waveguides enable remarkable waveguide sensitivity for both refractive index sensing and surface sensing. removal. In contrast, polymer-based EO modulators exhibit high-speed operation but suffer from process compatibility with the well-established silicon-on-insulator (SOI) technology, which makes high volume and cheap production challenging. After the invention by Almeida et al. in 2004 [1], slot waveguides became one key element to combine the well-established SOI technology with nonlinear optical polymers [2] . In recent years, there has been an increasing interest in slot waveguide structures. An SOI slot waveguide consists of a small gap in between two silicon rails, where a nonlinear optical polymer can be deposited. This narrow gap has two effects. First, the guided light is partly confined inside the gap. Second, the gap leads to an extremely large electric field, while voltages as low as 1 V are applied. Consequently, a record-high operation speed and large bandwidth with low energy consumption has been demonstrated using the silicon-organic hybrid (SOH) photonics [3] . Researchers have also discovered the advantages of slot waveguides for optical sensing. In this case, the slot waveguide takes advantage from the fact that more than 70% of the guided light can be confined near the silicon rails. Therefore, the light has a stronger interaction with the analyte compared to common strip waveguides, where only a fraction, 20%, of the light can be interacting. This strong light-analyte interaction leads to a large waveguide sensitivity, which has pushed the development of recent integrated optical sensors. This work examines the theory and applications of SOI slot waveguides for EO modulators and optical sensors. We provide a theoretical guideline for a deeper understanding of SOI slot waveguides. The present work is structured as follows. The following section focuses on a detailed theoretical description and analysis of slot waveguide structures. This is followed by a section on the fabrication scheme using standard SOI technology. Finally, application notes in electro-optics and optical sensing are given in Sections 4 and 5, respectively. Silicon-on-insulator slot waveguide This section deals with the basic concepts of SOI slot waveguides. For a deeper understanding of slot waveguide structure, the breakthrough paper of Almeida et al. [1] is recommended. However, the modal field expression given in [1] has some transcription errors and the given characteristic equation is not explicit for solving higher-order modes. Therefore, the correct modal field expression and the characteristic equation, which is also valid for higher-order modes is provided in this chapter, following the comprehensive work of Liu et al. [4] . Silicon-on-Insulator Slot Waveguides: Theory and Applications in Electro-Optics and Optical Sensing
doi:10.5772/intechopen.75539 fatcat:e6kovy62nfe35msgce5buuce5q