Introduction

Subwavelength-scale metallic structures are a basis for manipulating electromagnetic waves [693]. By engineering the geometry of individual structures and their coupling with each other and the environment, it is possible to construct materials that redirect radiation, couple freely propagating waves to highly localized modes and concentrate light into subwavelength-scale “hot spots.” At RF, these concepts have been developed to great maturity, where antenna and transmission line technologies have formed the basis for modern wireless communication [694]. It has been of recent interest to scale these concepts down to IR and even visible wavelengths, to create new functional materials that can be used in photonic and plasmonic circuits [40], field-enhanced spectroscopies [547], beam steering platforms [695] and new types of detectors [201].

Plasmonic nanostructures can be fabricated via two routes. The first is topdown lithographic fabrication, which employs well-developed techniques such as optical lithography, EBL and FIB milling [436]. The second is the chemical synthesis of colloids. NP synthesis dates back to Ancient Roman times where colloidal Ag and Au were used to color glass, famously exemplified by the Lycurgus Cup. Today, physical chemists can synthesize Au and Ag nanostructures with a broad range of shapes and sizes [696]. Top-down nanofabrication will continue to advance developments in nanophotonics, but it possesses intrinsic limitations. One is that the structures are defined in a focal plane and are typically planar. Another is that, for EBL and FIB, structures are written in series and limited to relatively small total areas.