Study of near-field circular dichroic optical forces on plasmonic nanostructures
- Author(s): Rajaei, Mohsen
- Advisor(s): Wickramasinghe, Hemantha Kumar
- et al.
Plasmonic nanostructures and nanoparticle assemblies with spatial symmetry breaking interact with right- and left-handed circularly polarized (RCP and LCP, respectively) lights differently. The difference between absorption of RCP and LCP by these nanostructures is called circular dichroism (CD). Compared to their molecular counterparts, broken symmetry nanostructures demonstrate orders of magnitude higher CDs, making them suitable for promising applications in photonic devices, such as circular polarizers and devices with negative refractive index, as well as in pharmaceutical sciences for enantioselectivity enhancement of chiral molecules. Although CD is useful information in characterization of these devices, since it is a far-field quantity, it cannot retrieve the underlying near-field interactions. Understanding the near-field interactions, however, is necessary in designing novel photonic devices with different applications. In this thesis, we investigate the near-field circular dichroic forces on plasmonic nanoparticle assemblies at nanoscale using photoinduced force microscopy (PiFM). We first introduce a PiFM technique based on gold nanoparticles (AuNPs) and silicon tip. Conventionally, gold-coated silicon tips are used in PiFM, which causes some anisotropy-induced distortions in the images because of the irregularities of gold grains on the silicon tip. This is especially important in CD studies as these asymmetries can create fake signals. Using AuNPs and silicon tip, we demonstrate much more accurate and distortion-free images, as well as an order of magnitude enhancement in signal-to-noise ratio. We then use this PiFM technique to study the near-field circular dichroic forces on different AuNP assemblies. We show that while two different mirror-symmetric (structurally achiral) assemblies can have zero CD, their near-field circular dichroic force maps can be different depending on their rotational symmetry. We finally demonstrate a metasurface, made of broken symmetry ramp-shaped nanostructures, which exhibits giant CD at the visible frequencies using a simple fabrication technique based on gradient focused-ion beam milling, addressing complicated fabrication challenges to create 3-D asymmetric nanostructures.