High Q-factor photonic cavities as new transducers for photothermal spectroscopy

Spectroscopic sensors represent a significant type of device designed to study the distinct fingerprint absorption spectrum of molecules. Photothermal spectroscopy (PTS) stands out as a highly sensitive and selective method, enabling the indirect measurement of the optical absorption of specific materials. In PTS, accurate measurement of refractive index changes is crucial. Therefore, suitable transducers for measuring such changes in gaseous and liquid samples are required. While excellent results have been achieved, the system remains bulky. The refractive index sensitivity and real-time measurement capabilities of integrated photonics make them ideal for interfacing with PTS systems. Integrated photonics enable the implementation of compact devices that facilitate efficient light-matter interaction. Among these devices, high-quality factor resonators can significantly enhance their performance by allowing light to circulate within the device for extended periods. Furthermore, they can be engineered to cover a wide range of refractive indices, making them versatile for various gaseous and liquid samples. Additionally, combining photonics with on-chip components such as microfluidics opens the possibilities for fully integrated lab-on-a-chip systems, providing comprehensive solutions for sample handling and analysis. In this thesis, various types of integrated photonic resonators have been designed and numerically studied for using as new refractive index transducers in PTS. The computational work conducted for this research required access to high-performance computing infrastructure provided by CINECA in Italy and Politecnico di Bari, Italy. Commercial solver RSOFT, and flexible open-source tools like MEEP and MPB, were employed. A combination of 2D and 3D-Finite Difference Time Domain simulations, as well as Rigorous Coupled Wave Analysis (RCWA) and other solvers, were used to simulate the proposed resonant structures. The combination of a reduced thermo-optic coefficient and low two-photon absorption has established Silicon Nitride as the primary platform for the devices proposed in this thesis. Photonic cavities with high Q-factors and small modal volumes offer increased sensitivity and lower detection limits. Consequently, this study prioritized the design of photonic crystal nanobeam cavities (PhCNC) and Micro Ring Resonators (MRR). MRRs are characterized by their Free Spectral Range (FSR), while PhCNCs can be optimized to support single-mode operation, which is particularly relevant for demonstrating a Hybrid External Cavity Laser (HECL). Fano resonances are a specific interference phenomenon observed in the scattering of waves. They are characterized by a spectral shape with an asymmetric line profile. The steep profile of Fano resonances can lead to more significant signal changes when a transducer interacts with samples. To achieve Fano resonance lineshapes, a photonic crystal nanobeam was sidecoupled to a MRR. Three variations of PhCNCs were investigated. The design with angled sidewalls demonstrated high tunability of the Q-factor, favoring more asymmetric cladding configurations. In the case of elliptical nanopillar design, there was a reduction in the impact of fabrication errors on the Q-factor. The Silicon-based slotted configuration exhibited a significant enhancement in the Q-factor through sidewall engineering. Furthermore, by side-coupling a S-bent waveguide to the PhCNCs, nearly single-mode operation was achieved. Conversely, the designs of circularhole and rectangular-slot Fano resonators exhibited Fano lineshapes consistently. Furthermore, the ability to tune these asymmetric shapes was successfully demonstrated. Additionally, metasurfaces for Silicon on Si3N4 membranes operating in Mid-IR wavelength range are explored. Near-field enhancement and Fano-like features were examined. Based on the simulations conducted, the optimal and most rugged geometries that can be fabricated in a single etching process were found. The fabrication was carried out by our consortium partners at Tyndall National Institute and the Centre of Advanced Photonics & Process Analytics (CAPPA), Ireland. The fabrication was led by Simone Iadanza and Artem Vorobev. Initial characterization of the fabricated devices and some of the experiments were conducted in collaboration with the CAPPA lab, including the demonstration of the HECLbased PhCNC sensor. The fabrication of metasurfaces is an ongoing process. At TU Wien, Vienna an experimental setup for the characterization of the fabricated Photonic integrated circuits was built from scratch. The experimental measurements exhibited agreement with the trends observed in simulations. Nevertheless, certain discrepancies are noted and discussed. To enhance stability and enable the measurement of small refractive index changes, thermal stabilization control into the setup was incorporated. Furthermore, advancements in the integration of liquid handling through microfluidics are demonstrated. The collaboration with the Dipl.-Ing. Silvia Schobesberger, a member of the Cell Chip group at TU Wien, allows the realization of a reliable microfluidic cell. The implemented microfluidic cell on the refractive index transducer Si3N4 surface was tested with aqueous glucose solutions showing an almost linear response. Thus, this transducer can now be used as a refractive index detector in liquid chromatography and in flow injection analysis. On the other hand, ongoing efforts on Photo Thermal Spectroscopy concerns about the refractive index transducers presented here. Therefore, further improvements toward an integrated PTS sensor could build upon the results discussed in this thesis.