| dc.description.abstract | The field of plasmonics has drawn a considerable amount of research interest for the past 20 years, and now, plasmonics is a vital part of nanophotonics. Numerous applications have been enabled by plasmonic structures in a wide range of areas, including engineering, medicine, biology, food science, and environmental science. Among all the applications, the field of plasmonic sensing has made remarkable progress, and it continues to grow quickly. Plasmonic sensors, empowered by cutting-edge nanofabrication techniques, are offering label-free and robust sensing performance. In order to apply the plasmonic sensor technology to even more areas and real-world problems, one needs to optimize and improve the sensor technology toward realizing low-cost, portable, and high-performance sensors that can operate in unstable environments.
In this thesis, we propose and fabricate several nanostructure-based plasmonic sensors to improve performance in variable environmental conditions and reduce the cost of characterizations. The first two sensors are based on metallic two-dimensional nanograting that can create high-quality factor resonance features in visible wavelengths. Both sensors have the ability of self-referencing, which makes them suitable for working in an unstable environment. Further, both sensors are highly sensitive
to the small changes in the local refractive index and are also capable of detecting surface attachments. Last but not least, both sensors have simple structures resulting in ease of fabrication and operate in visible and near-infrared regions which makes them excellent candidates for low-cost applications. To demonstrate the sensors, we design and numerically evaluate the performance of the proposed structures using Rigorous coupled-wave analysis (RCWA) and Finite-difference time-domain (FDTD) methods. We also investigated the effect of geometrical parameters on the performance of the sensors and demonstrated that a photonic designer had many degrees of freedom to design for the proposed devices to optimize the sensors for diverse applications. Secondly, we fabricate the designed structures using nanofabrication techniques such as electron beam lithography (EBL) and lift-off, and we experimentally confirm the different plasmonic modes that are excited in the sensor. We also optimize the sensors to achieve desirable results. Finally, we characterize the fabricated sensors and experimentally evaluate them in terms of sensitivity. The experimental results agree with the simulations, and the sensors showed high performance as predicted during the design and simulations. Based on the experimental results, the sensors can generate several resonance features in the visible to
near-infrared range, in which at least one of these resonance modes is very sensitive to the changes inv the surrounding refractive index and can be used as a sensitivity measurement point. On the other hand, at least one of the resonance features is isolated from the surrounding environment and can be used as a self-referencing point. We propose that using self-referencing, changes due to temperature can be extracted. For some designs, the sensitivity values achieved in the experiments are even higher than the values that were predicted in the simulations. The fabricated sensors showed lots of potential for realizing a low-cost platform for self-referenced plasmonic sensors. The third proposed plasmonic sensor, designed and fabricated in this work, is a hybrid platform based on titanium dioxide (TiO2) nanowire arrays integrated with plasmonic layers. The sensor can create a very sharp resonance feature in reflections in the visible range, resulting from the coupling of plasmonic modes and nanowire optical modes. While similar resonances have been demonstrated before in the transmission, e.g., with nano-hole plasmonic arrays, to our knowledge, this is the first demonstration of single peaked reflections. This results in the generation of very vivid structural colors. After designing and evaluating the proposed design using FDTD and RCWA simulations, we fabricate the nanowire arrays using the dry etching technique and achieve highly ordered nanowire arrays. Further, the structure is sensitive to the changes in the surrounding refractive index, which makes it suitable for realizing low-cost colorimetric sensors requiring only a camera and image processing instead of a spectrometer. This will appreciably reduce the cost of a sensing system. | en |
