10.1117/2.1201208.004366 Gain-enhanced nanoplasmonic metamaterials: ultrafast sub-wavelength emitters Ortwin Hess and Kosmas Tsakmakidis Gain media have been efficiently integrated into the fabric of nano- plasmonic metamaterials, removing losses and enabling ultrafast light sources at nanoscopic dimensions. Nanoplasmonics and optical metamaterials have in the last 10–15 years emerged as a new paradigm in condensed matter optics and nanoscience, offering a fresh perspective on the world of optics. They enable efficient coupling of electromagnetic fields to the nanoscale: the world of biological and nonbiological molecules. 1 This tight localization of light to truly nanoscopic dimensions—well below the diffraction limit for visible light— enhances its interaction with matter, paving the way for a mul- titude of classical and quantum nano-optics applications. How- ever, metal optics suffer from inherent dissipative losses, which have persistently hampered many of the envisaged uses. Ad- vances in the theoretical understanding and experimental fab- rication of gain-enhanced nanoplasmonic metamaterials now promise to overcome these hindrances, potentially leading to novel nanophotonic components and devices. Dissipative losses in nanoplasmonics arise from the interac- tion of incident photons with quasi-free conduction electrons in metals, and therefore constitute an inherent feature of the responses of metal-based nanodevices. For truly sub-wavelength plasmonic structures, these losses follow universal laws, i.e., they do not depend on a particular geometric configuration, but rather only on the type of (usually noble) metal used. Typical damping rates ( ) are of the order of   100ps 1 , requiring gain coefficients  =c  10 3 –10 4 s 1 to compensate for the losses. As an alternative strategy, optical metamate- rials pave the way toward nanoscale control of light at the fabrication level. However, such control needs to be modulated dynamically, on-demand and in real time. Both of the above- mentioned challenges can be met by metamaterials with gain incorporated directly into their fabric. Figure 1. Schematic of a gain-enhanced plasmonic nano-fishnet meta- material together with example profiles of the inversion (lower left) and electric-field amplitude. h m , h c , and h d denote the height of the metal, cladding, and dielectric layers, respectively, a x and a y are the width of the rectangular holes in the x and y direction, and p is the periodicity of the material. Pioneering theoretical 2, 3 and experimental 4 work has recently shown that it is realistically possible to overcome dissipative losses of nanoplasmonic metamaterials, even in the exotic negative-index regime. However, whether steady-state net amplification can be accomplished is yet to be answered. An- other question is what kind of active nanocomponents can we ultimately expect by enhancing metamaterials and nanoplas- monics with gain? Continued on next page