Integrated Optical Sensors utilizing Slow-light Propagation in Grated-Waveguide Cavities

Abstract Integrated Optical Sensors utilizing Slow-light Propagation in Grated-waveguide Cavities Owing to the small size of integrated optical (IO) devices many basic functions can be integrated on one single IO chip. IO sensors are suitable candidates for accurate detection of small changes of physical or chemical parameters. The integration offers advantages such as enabling a high density of functionalities, automatic and stable alignment of elements, a high potential for mass production with in principle low production costs, and the possibility for the realization of sensor arrays for multi-parameter detection. The main goal of this PhD project is firstly to design, fabricate and demonstrate functioning IO devices based on grated waveguides for sensing applications. A grated waveguide (GWG) is a waveguide with a finite-length grated section, being a structure with a periodic variation of the dielectric constant. Such a structure acts as both a 1-dimensional photonic crystal (PhC) and, owing to modal reflections at the waveguide-GWG transitions, an optical resonator, as evidenced by fringes in the transmission spectrum. In particular near the band edge these fringes can be extremely sharp, which is related to both the near band edge shape of the dispersion curve, corresponding to slow light propagation, and high modal reflectance due to mode mismatch between WG and GWG modes. Both effects lead to strong light-matter interaction, which can be exploited for sensing applications. In this thesis, we demonstrate the versatility of a silicon nitride GWG optical cavity as a compact IO sensor for bulk-index concentration sensing, label-free protein sensing and mechano-optical gas sensing. For concentration sensing, the sensing principle is based on the bulk index change of the GWG top cladding. The principle of the label-free protein sensing relies on the growth and measurement of an ad-layer on the GWG surface, owing to the antigen-antibody interaction. The mechano-optical gas sensing is based on stress-induced deflections of a microcantilever (μCL) suspended above the GWG, which are due to H2 gas absorption by the palladium receptor layer coated on the μCL surface. In the first chapter of this thesis an overview is given of bio- and gas-sensors. In chapter 2, the background of slow light propagation in GWGs and its utilization for sensing applications are discussed. In chapter 3, results related to the first 2 sensing applications (concentration sensing and label-free protein sensing) are presented; here, sensitivity and limit of detection of the sensors are analyzed in detail. The design and fabrication of the GWG-CL integrated readout, and the demonstration of the integrated mechano-optical sensor for gas sensing, are presented in chapter 4 and chapter 5, respectively. Results of an optimization study of the integrated mechano-optical readout principle, on the basis of numerical calculations, is presented in chapter 6. In chapter 7, conclusions and outlook, based on the results presented in this thesis, are given.