Research on Bulk-Silicon-Based Photonic Waveguides in the Mid-Infrared

Abstract

Many molecules have unique absorption spectra due to the vibrational characteristics of molecular bondings such as C-H, N-H, O-H, C-C, N-O, and C=O. The absorption spectrum is located in the mid-infrared (mid-IR) range including wavelengths from 2 μm to 10 μm. Since a molecule can be detected through an analysis of its absorption spectrum, it is very important to develop photonic sensors, especially waveguide-based sensors, operating in the mid-IR. Recently, there have been many researches on waveguide-based sensors made of a silicon-on-insulator (SOI) wafer. However, SOI substrates are not suitable for photonic waveguides in the wide mid-IR since SiO2 strongly absorbs light with a wavelength longer than 3.6 μm. To overcome this limitation, diverse photonic waveguides in the mid-IR have been demonstrated. In this thesis, a bulk-silicon-based photonic waveguide is theoretically and experimentally studied. It is made of a bulk-silicon wafer by using a simple fabrication process. Light is guided through a strip with an inverted-triangular cross-section, which is supported on a bulk-silicon wafer. The bulk silicon-based photonic waveguide can be used in the wide mid-IR because of no use of SiO2, and photonic sensors based on it can be realized at a low cost. After brief explanation of the background of this work, the fabrication process of the waveguide is explained. Then, the bulk-silicon-based photonic waveguide is designed and theoretically analyzed. Next, the realized bulk-silicon-based photonic waveguide is experimentally investigated. The measured propagation loss of the fabricated waveguide at a wavelength of 1550 nm is 4 dB/cm. This waveguide would be useful as a chemical sensor, which can be realized with a low cost.