Advanced nanophotonics for gas spectroscopy

This thesis introduces advanced nanophotonic integrated devices aimed at improving minia-turized, cost-effective multi-gas detection and on-chip spectroscopic systems. Traditional spectroscopic techniques often require bulky optical components and multiple detectors, limiting their scalability for multi-gas sensing. The proposed integrated duplexers and tri-plexers enable switching between lasers to detect multiple gases using a single system. The work focuses on the design and optimization of broadband angled multimode interference duplexers, directional coupler-based duplexers, and cascaded directional coupler-based tri-plexers for combining spectroscopically relevant wavelengths in the near-infrared region. The target gases include ammonia, methane, and carbon dioxide. Through comprehensive simulations and experimental investigations, the proposed on-chip designs demonstrate su-perior performance compared to existing solutions and have a unique advantage in terms of smaller footprint and improved coupling efficiency. DC-based duplexer has been success-fully integrated with laser and GRIN lens components, resulting in a ready-to-use module for multi-gas sensing applications. A semi-integrated photonic sensing system is presented, exploiting on-chip waveguides with Quartz Enhanced Photoacoustic spectroscopy and Light induced thermoelastic spec-troscopy (LITES). Side-polished optical fibers are explored to enhance light-matter interac-tion path when detecting water vapor and methane gases using LITES method. To further improve integration of integrated nanophotonic devices with spectroscopic devices and to enhance light-matter interaction, a novel wave confinement approach is introduced using high-contrast grating hollow core waveguides. These waveguides feature a reflective sur-face that maintains high transmission while allowing gas flow through the sidewalls, mak-ing them particularly suitable for gas spectroscopic applications. They are specifically op-timized for methane sensing at a wavelength of 3.27 μm. The final goal of this thesis is to develop a complete system that integrates a multiplexer with integrated lasers and high-efficiency interaction pathways, such as hollow core waveguides, into a spectroscopic de-vice. This compact and integration-friendly design holds great promises for enabling the development of portable, high-precision, and real-time multi-gas sensing devices for appli-cations from industrial, agricultural to environmental monitoring.