Three dimensional nanoplasmonic surfaces : modeling, fabrication and characterization

Abstract

Today designing functional nanoplasmonic structures specific to a variety of applications attracts great interest from various fields ranging from optoelectronics to life sciences. There are numerous ways of making nanoplasmonic structures. Among them, nanopatterning of a thin-film metal layer is one of the most common approaches, which allows for finely controlled fabrication of a plasmonic unit and their repeating layout in the plane of the starting metal film. Although there are many examples of such nanopatterned plasmonic structures reported to date, they are typically designed and implemented on a planar surface. In these architectures, plasmonic layout commonly covers significantly less than 100% of the substrate surface and can provide field localization most strongly around the sharp corners and small gaps between the patterns. In the case of using a periodic layout, which is commonly employed for experimental realization (although periodicity is not necessary), the plasmonic array inherently yields a duty cycle substantially less than unity (usually close to 0.5). As a result, the surface coverage of nanopatterned plasmonic structures on a planar surface has intrinsically been limited and the field enhancement across their nanoplasmonic layout has been possible mostly in the plane and slightly above it. To address these limitations, this thesis proposed and demonstrated three-dimensional (3D) nanoplasmonic arrayed structures designed and implemented on a non-planar surface that allows for strong field enhancement in the out-of-plane direction and enables a very large surface coverage of the substrate close to unity. The thesis work included both numerical modeling and experimental characterizations. As a proof-of-concept demonstration, we fabricated non-planar arrays of checkerboard nanostructures, each with two-fold rotational symmetry, laid out in a volumetric fashion as two interlocked square lattice arrays at two different levels, facilitating strong field localization vertically between these two complementary planes. The resulting nanofabricated samples exhibited a maximum surface coverage of 100% in plan view. With full electromagnetic solution of such 3D nanoplasmonic surfaces, we showed that the out-of-plane field localization is 7.2-fold stronger than the inplane localization, in comparison to their two-dimensional (2D) components alone. These numerical results agree well with the experimental observations including far-field optical transmission and reflection measurements. The absorption spectroscopy further revealed that the resulting spectrum of the 3D checkerboard features a unique signature arising from the out-of-plane localization, which does not exist in the case of the 2D counterparts. These results indicate that 3D nanoplasmonics of such non-planar surfaces provides us with the ability to generate and better utilize the plasmonic volume, possibly useful for increased plasmonic coupling and interactions.