Plasmonic and diffractive coupling in 2D arrays of nanoparticles produced by electron beam lithography

Nanofabrication is one of the driving forces leading to developments in a variety of fields including microelectronics, medicine, and sensors. Precise control over nanoscale architecture is an essential aspect in relating new size-dependent material properties. Both direct write methods and natural lithography's offer a unique opportunity to fabricate "user-defined" writing of nanostructures in a wide range of materials. Electron Beam Lithography (EBL) and Nanosphere Lithography (NSL) provide the opportunity to fabricate precise nanostructures on a wide variety of substrates with a large range of materials. Using electrodynamics calculations, Schatz and coworkers have discovered one and two dimensional array structures that produce remarkably narrow plasmon resonance spectra upon irradiation with light that is polarized perpendicularly to the array axis. In order to investigate these interactions, precise control of nanoparticle orientation, size, shape and spacing is necessary. If the overall structures have excessive defects then the effect may not be seen. For the two dimensional arrays, to have the best control over array fabrication and to look at these interactions experimentally, EBL was used to construct both hexagonal arrays of circular cylinders and the Kagome lattice. The interparticle spacing in each of these structures was varied systematically. Dark field microscopy was used to look at overall sample homogeneity and collect the single particle plasmon resonance spectrum. Additionally, both dark-field and extinction spectroscopies were used to look at the bulk spectral properties of each array type and each spacing. In investigating of the two dimensional arrays, the Kagome structure was also compared to samples produced by traditional NSL to study the optical interaction of defects, domains, and overall sample uniformity on the shape and location of the plasmon resonance. This work illustrates a deeper understanding in the nature of the optical coupling in nanostructures and this knowledge can be utilized in the future to fabricate designer (tailor made) substrates for plasmonic and surface-enhanced raman applications.