| dc.description.abstract | Near to mid infrared (NIR-MIR) chemical sensing is a remarkably influential analytical technique that capitalizes on the interaction between matter and infrared radiation within a specific wavelength range. Predominantly rooted in spectroscopy, this method exploits the unique vibrational modes of molecules when excited by infrared light, yielding distinct absorption or scattering patterns. Its versatility lends itself to various applications, encompassing pharmaceuticals, environmental monitoring, food safety, and material characterization. This technique stands out due to its ability to provide rapid, non-destructive analysis, facilitating the identification and quantification of diverse substances, ensuring product integrity, and assessing environmental conditions. Its sensitivity and capacity to offer molecular insights make it invaluable across industries and scientific research domains. The fingerprint region of hazardous gases like Carbon dioxide (CO2), Nitrous oxide (N2O), Nitrogen dioxide (NO2), Nitric oxide (NO), Ammonia (NH3), Ethylene (C2H4), Acetylene (C2H2), Hydrogen Cyanide (HCN), and Methane (CH4), makes the sensing in the near to mid infrared region quite promising.
This work involved the design of an absorption-based sensor using a polarization-sensitive slot waveguide on the Germanium on insulator (GeOI) platform. We harnessed the wide transparency of Germanium in the mid-infrared region, allowing the evanescent field to facilitate light-matter interaction. This research also introduced a novel polarization-independent, broadband trapezoidal bi-layer grating structure designed for Near to Mid-Infrared applications on two different platforms. The Germanium on Insulator (GeOI) grating structure exhibited significant bandwidth and high efficiency for both TE and TM modes. In contrast, the Silicon Nitride on Insulator (SiNOI) grating structure also demonstrated substantial bandwidth but with varying transmission efficiencies for TE and TM modes at a different wavelength. Broadband grating couplers play a critical role in absorption-based slot waveguide sensors by facilitating the efficient channelling of light to the slot waveguide. These couplers are essential components as they enable the precise interaction of light with the sensor's sensitive region. Their broadband characteristics are particularly important, ensuring that a wide range of wavelengths can be efficiently coupled to the slot waveguide, enhancing the sensor's versatility and applicability across different spectral ranges. In essence, broadband grating couplers act as optical interfaces, efficiently transferring incident light to the slot waveguide, where it interacts with the sample being analyzed. This process is crucial for the sensor's functionality as it directly influences its sensitivity and ability to detect and quantify target molecules or substances. Therefore, the development and optimization of broadband grating couplers are of paramount importance in the design and performance of absorption-based slot waveguide sensors, enabling them to meet the demands of various sensing applications across different wavelengths. Furthermore, our research resulted in the development of a versatile polarization converter, enabling seamless bidirectional transformation between Transverse Electric (TE) and Transverse Magnetic (TM) fundamental modes. This innovation is pivotal for achieving precise control over light polarization in integrated photonic circuits, unlocking a wide range of potential functionalities and applications. This cutting-edge device displayed remarkable sensitivity as a multianalyte absorption-based sensor, capable of detecting hazardous gases at trace levels, even down to parts per billion (ppb). Its potential for real-world environmental and safety applications was evident. This structure which not only offered a more compact footprint compared but also provided an effective method for label-free detection of target
molecules. Its on-chip integration potential made it particularly promising for applications in industrial and environmental monitoring, health analysis, and food processing, surpassing traditional spectrometers like Fourier Transform Infrared Spectroscopy (FTIR).
We also have proposed a tapered structure on the Germanium-on-Insulator (Ge-OI) platform. Our approach involves the careful optimization of multiple parameters to achieve outstanding coupling efficiencies. The main focus of this design is to ensure high performance for both TE (Transverse Electric) and TM (Transverse Magnetic) polarizations, making it versatile and effective across a broad spectrum ranging from 2.5 to 3.5 micrometers. This exceptional performance is maintained irrespective of the polarization state. Our tapered structure represents a significant advancement in the field of integrated photonics and optical components. It offers coupling efficiencies of over 80% for TE polarization and more than 40% for TM polarization, which is a remarkable achievement. This level of performance ensures that the structure can efficiently handle light across the entire specified spectrum, making it a highly valuable and versatile component for a wide range of photonic applications. In summary, our work on the optimized tapered structure on the Ge-OI platform demonstrates its capability to maintain excellent coupling efficiencies for both TE and TM polarizations across a wide spectral range. This design is a significant contribution to the field of photonics and opens up new possibilities for advanced optical components and devices.
With respect to this, we have also developed a refractometric sensor featuring a perovskite-based functional layer designed for the rapid and precise monitoring of NO2 in the near-infrared (NIR) region. Our research has revealed that this sensor can achieve an impressive effective index variation at the level of 10^-4, showcasing its high sensitivity to changes in the analyte's refractive index. One of the key strengths of this sensor is its remarkable reversibility, which means it can consistently provide accurate measurements over time. Moreover, it boasts an extended shelf life, ensuring that it remains reliable for an extended period. Additionally, the sensor demonstrates a high level of selectivity, allowing it to specifically target and detect NO2 while minimizing interference from other substances. Overall, our findings underscore the effectiveness and potential applications of this ultrafast refractometric sensor for NO2 monitoring, making it a valuable tool for various environmental and industrial contexts.
Additionally, this research introduced a refractometric sensor based on a Distributed Bragg Reflector (DBR) comprising alternate layers of Silicon Oxide or Silicon Nitride. This sensor exhibited good sensitivity to changes in refractive index. Experimental results confirmed the trends predicted by simulations, particularly when Poly methyl methacrylate (PMMA) was used as the analyte. This practical demonstration emphasized the feasibility and potential of employing periodic structures for refractive index-based sensing. These findings have significant practical implications, particularly in the fields of photonics and chemical sensing. Such refractometric sensors can find applications in various industries, including environmental monitoring, biomedical diagnostics, and material characterization, where the accurate measurement of refractive index changes is essential for detecting and quantifying specific substances or conditions.
Then we also proposed and demonstrated a GMR based Sensor based on Silicon on insulator (SOI) platform, our practical work involved experimental investigations, including the successful fabrication and characterization of various periodic structures. These experiments validated our theoretical findings and leveraged Modulated Beam Moving Stage (MBMS)-assisted e-beam lithography. Designing cavities in optics and photonics has been a subject of extensive exploration due to its critical role in both practical applications and fundamental research. Guided-mode resonance (GMR) structures offer excellent potential for optical sensing due to their sensitivity to the surrounding environment and their ability to produce high-quality factor resonances. These structures typically
comprise a combination of a waveguide and a grating, which serves to couple incident light into the waveguide. The resonance in GMR occurs when the waveguide's guided mode aligns with the diffracted mode generated by the grating. The specific characteristics of this resonance are determined by the properties of the grating, the waveguide, and the surrounding medium. One notable advantage of GMR-based sensor systems is their potential to be directly excited by external light sources, eliminating the need for specialized coupling mechanisms. This characteristic place GMR-based sensors at an advantage over conventional counterparts, which often suffer from coupling losses and limited light-matter interaction volumes. | en_US |
