Scalable chiral plasmonic structures
Larsen, George Keefe
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As chiral metamaterials become increasingly more technologically relevant, scalable yet proficient nanofabrication methods will be needed for their production. Dynamic shadowing growth (DSG) takes advantage of the shadowing effect during physical vapor deposition and is a simple and powerful tool to produce chiral nanostructures. This dissertation describes several new DSG strategies for scalable production of chiral plasmonic thin films with significant optical activity in the visible and near-infrared wavelength region. The helix is the prototypical chiral structure, yet fabrication of nanoscale helices is a significant technological challenge. By using metal composite (Ti/Ag) and metal/dielectric composite materials (Ag/MgF2), it is demonstrated that nanoscale helices can be fabricated through DSG at room temperature in a scalable manner. However, these thin films have a high degree of alignment, which increases the difficulty of interpreting the optical measurements. Thus, a generalized ellipsometry measurement scheme is introduced to extract the optical parameters from aligned films. It is found that some of these DSG helical structures are among the most optically chiral films to date. It is also shown that self-assembled colloidal monolayers of nanospheres can serve as effective templates for the production of a wide variety of highly chiral films. Furthermore, this method can be combined with chemical techniques to dynamically tune the optical response. These chiral films can have coatings with well-defined shapes that are analyzed using a continuous chirality measure. This measure of chirality allows for the quantification of chirality, providing insight into the origin of optical activity in these films. Finally, DSG films are developed to serve as both surfaced enhanced Raman scattering (SERS) substrates and as molecular chirality sensors through chiral plasmonic sensing. A molecule’s “handedness” determines its function in the body, and introduction of the wrong-handed molecule can have serious consequences. Therefore, a sensor that is sensitive to molecular chirality is highly desirable. In addition to their SERS response, the DSG films are investigated for their sensitivity to the chirality of L- and D- enantiomers of tryptophan. Notably, the results show that the measured optical response depends on both the handedness of the film and the handedness of the molecule.