Plasmon field effects on the operation of optical biosensors

Optical biosensors have emerged as powerful tools in the field of medical diagnostics due to their high sensitivity and specificity. Significant advances have been enabled by plasmonic features, which modify light-matter interactions in subwavelength volumes using metallic nanostructures. In this thesis, we tune the field distribution associated to plasmonic biosensors to address the limitations of the current optical biosensors in terms of novelty, miniaturization, and sensitivity. We introduce a novel technique called distance-controlled surface-enhanced Raman spectroscopy using gold nanospheres, which studies the spatial origin of Raman signals in layered biological particles, such as extracellular vesicles. Additionally, we develop a high-performance, grating-based surface plasmon resonance (SPR) sensor using a tunable laser at normal incidence. Our research also investigates SPR mode splitting at quasi-normal incidences and optimizes grating to avoid such splitting in our SPR system. Furthermore, we demonstrate grating-based, plasmon-enhanced upconversion luminescence (UCL) for highly sensitive biosensing. The enhanced local fields significantly increase the absorption rate of upconverting nanoparticles, leading to UCL enhancement of up to 65 times. This advancement enables UCL-based assays with high signal-to-noise ratios, thus improving sensitivity and reducing the required excitation power in scanning-free imaging systems. Our presented plasmonic biosensors also offer cost-effective and scalable fabrication methods such as chemical synthesis for gold nanospheres and nanoimprinting for gratings. This thesis opens different ways to advance optical biosensors towards diagnostics applications.