Engineered plasmonic and all-dielectric nanostructures for improved optical biomolecular sensing

The size of typical chemical and biological molecules is in the nanometer scale, ranging from a few nanometers for DNA double helix to ~100 nm for virus. For optical detections of these molecules, an optical microscope is not the most effective in terms of resolving molecules in close proximity in subwavelength scales. Extreme light concentration into a nanoscale volume that is comparable to the size of target molecules can greatly improve the performanace of biomolecular sensing. For this matter, metal nanostructures that support localized surface plasmon resonance (LSPR) have extensively been utilized as plasmonic biosensors for specific label-free bioanalysis. In the first part of this thesis, the sensing performance based on plasmonic resonance shift is investigated for 2D periodic arrays of metal nanoparticles. Such periodic plasmonic arrays exhibit extremely sharp resonances, which have been assumed to provide lower limit of detection. However, diffractive coupling between plasmonic nanoparticles in the array greatly alters the surface electric field distribution, resulting in variations of biosensing performance. Two types of sensitivity are clarified, i.e. the bulk sensitivity and surface sensitivity. A reliable method based on a new definition of surface sensitivity is proposed for more accurate biosensing evaluation. Protein bilayer detections are carried out with an improved design of the 2D plasmonic array. It is demonstrated that the commonly used figure-of-merit (FoM) in the literature could not be precise as a benchmark for overall performance assessment. Other photonic coupling effects are also systematically studied and compared. Plasmonic biosensor design and evaluation are suggested based on the specific biological targets under study. Furthermore, combining LSPR with Raman spectroscopy, a novel type of surface enhanced Raman spectroscopy (SERS) substrate is developed based on 300 mm wafer-level silicon nanopillar arrays. It features uniform and ultra-dense arrays of Au nanoparticles with sub-10 nm gap size in between, giving rise to strong and reproducible molecular SERS signals. At the end of the thesis, an alternative material, namely high-index amorphous silicon, is approached for its novel optical manipulations. A wavelength router in the visible range is designed with bidirectional scattering into diametrically opposite directions, which can be effectively controlled by the incident wavelength. The unique scattering properties are rooted in the strong magnetic dipole resonance in the visible range and the smart phase design of the electromagnetic multipole modes. Comparisons with similar gold design demonstrate that the silicon exhibits stronger scattering cross section with minimal absorption losses. This will open up exciting possibilities for further optical design in biosensing applications.nrpages: 137status: publishe