On-Chip Photonics for Spectroscopy and Sensing

Abstract

On-chip photonics for spectroscopy and sensing refers to the integration of photonic components and devices onto a single semiconductor chip to enable efficient and compact spectroscopic and sensing applications. Traditional spectroscopy setups often involve complex optical systems with bulky components, limiting their portability and integration with other technologies. On-chip photonics seeks to address these challenges by leveraging the miniaturization and integration capabilities of semiconductor technology. This emerging field encompasses various on-chip devices, such as waveguides, resonators, filters, and detectors, designed to manipulate and analyze light for spectroscopic purposes. The integration of these components allows for enhanced light-matter interactions, precise control of optical signals, and improved sensitivity in detecting spectral features. Applications of on-chip photonics for spectroscopy span a wide range, including chemical sensing, environmental monitoring, medical diagnostics, and telecommunications. The compact nature of on-chip devices enables the development of portable, cost-effective, and highly efficient spectroscopic systems. Overall, on-chip photonics for spectroscopy holds great promise for revolutionizing the way we perform spectral analysis, making it more accessible, scalable, and suitable for integration into diverse technological platforms.

This thesis encompasses a comprehensive exploration of material platform selection for on-chip spectroscopy, emphasizing the critical optical and material characterization techniques required for validating material suitability. Fundamental aspects of waveguides and light-chip couplers are elucidated, covering design, fabrication, and characterization processes tailored for both visible and near-infrared wavelengths. Further, the alteration in spontaneous emission of an emitter near a waveguide is detailed, revealing the complex relationship between emitter orientation and waveguide propagating modes. Experimental demonstrations showcase two key schemes in optical spectroscopy and quantum optics: evanescent excitation coupled with top collection and top excitation coupled with evanescent collection. These schemes play a pivotal role in probing and manipulating nanoscale entities, illustrating their practical application in advancing on-chip spectroscopy and quantum phenomena. The focus then shifts to the utilization of silicon nitride waveguides for on-chip fluorescence applications. Two efficient fluorescence signal collection methods, top collection, and evanescent collection are demonstrated.

Subsequently, we delve into the design optimization, fabrication, and optical characterization of Guided Mode Resonance (GMR) structures designed for light amplification and collection. We achieve a significant 200% amplification of light using a two-dimensional GMR structure. Additionally, a novel approach for Raman signal enhancement using a planar-guided mode resonator is introduced. Experimental demonstrations highlight a remarkable 4-order enhancement achieved with the resonance grating structure, positioning robust and planar dielectric resonators as competitive alternatives for enhanced Raman spectroscopy platforms.

We also explored fluid sensing at telecom wavelengths using double-slot waveguides. Our experimental demonstration begins with showing an efficient and compact method for light coupling between a strip waveguide and a double-slot waveguide. We verified the excitation of the double-slot mode by analysing the spectral response obtained from an asymmetric Mach-Zehnder Interferometer (MZI). We achieved highly efficient light coupling to a double-slot waveguide, showcasing an ultralow insertion loss of 0.025 dB/coupler and a propagation loss of 6.17 ± 0.6dB/mm. To demonstrate refractive index sensing, a silicon double-slot waveguide in an asymmetric MZI configuration is employed. The spectral response of the MZI is tracked using an optical spectrum analyser to monitor refractive index changes of Potassium Chloride solutions. We experimentally measured a sensitivity of 700 nm/RIU and a LOD of 2.85 X 10−5 RIU using a detector with a system resolution of 10 pm. Also, we attribute that a Limit of Detection (LOD) of 7.142 X 10−6 RIU can be achieved by employing a detector with a system resolution of 5 pm. To our knowledge, we report the best-in-class sensitivity and LOD for a silicon waveguide-based system. We also report an intrinsic LOD of 1.42 X 10−3 RIU.