The discovery of the enhancement of Raman scattering by molecules adsorbed on nanostructured metal surfaces is a landmark in the history of spectroscopic and analytical techniques. Significant experimental and theoretical effort has been directed toward understanding the surface-enhanced Raman scattering (SERS) effect and demonstrating its potential in various types of ultrasensitive sensing applications in a wide variety of fields. In the 45 years since its discovery, SERS has blossomed into a rich area of research and technology, but additional efforts are still needed before it can be routinely used analytically and in commercial products. In this Review, prominent authors from around the world joined together to summarize the state of the art in understanding and using SERS and to predict what can be expected in the near future in terms of research, applications, and technological development. This Review is dedicated to SERS pioneer and our coauthor, the late Prof. Richard Van Duyne, whom we lost during the preparation of this article.

We present experimental evidence of sharp spectral features in the optical response of 2D arrays of gold nanorods. A simple coupled dipole model is used to describe the main features of the observed spectral line shape. The resonance involves an interplay between the excitation of plasmons localized on the particles and diffraction resulting from the scattering by the periodic arrangement of these particles. We investigate this interplay by varying the particle size, aspect ratio, and interparticle spacing, and observe the effect on the position, width, and intensity of the sharp spectral feature. DOI: 10.1103/PhysRevLett.101.143902 PACS numbers: 42.25.Fx, 73.20.Mf Nanoparticles of noble metals have been the subject of many detailed studies due to their unique optical properties [1], in particular because they can support localized surface plasmon resonances (LSPR) [2]. The LSPR associated with metal nanoparticles exhibit a high degree of optical field confinement [3,4], together with a high sensitivity to their local environment. Where two or more metallic nanostructures are in close proximity, the possibility exists for interaction between the modes of the individual nanostructures to form new hybrid modes [5]. The case of two interacting particles has been extensively studied (for example, see [6]). For multiple nanostructures there is also the possibility of coherent interaction arising from multiple scattering. Light that is scattered so as to propagate in the plane of the particles will undergo multiple scattering by the regularly spaced particles. A geometric resonance arises when the wavelength of the scattered light is commensurate with the periodicity of the array, which, when it occurs in the same spectral range as the LSPR, may lead to a dramatic modification of the measured optical extinction. It appears that this effect was first predicted by Carron et al. (see, for example,[7]) and Markel [8,9] and more recently followed up by Zou et al. [10]. Further theoretical or computational work by these groups has extended our understanding [8][9][10][11][12][13], and a tutorial review linking these concepts to those associated with hole arrays has recently been given [14].Experiments to confirm the existence of these sharp diffractive features in the optical response of metallic nanoparticle arrays have met with only limited success. Haynes et al. [18] performed detailed studies of arrays of gold and silver nanoparticles, but the effect was not as pronounced as expected from the modeling. In each case, failure to observe the sharp spectral features appears to be due to one or more of the following factors: lack of an homogeneous environment, an angle spread of the illumination, an inappropriate choice of the particle volume and aspect ratio. Félidj et al. [19] reported sharp features in a system consisting of a regular array of gold nanorods supported on a thin indium tin oxide (ITO) layer. However, the presence of the ITO layer complicates the analysis and makes the underlying physics harder to unrav...

Noble-metal nanoparticles [1] with localized surface-plasmon resonances (LSPR) have been recently used to prepare new materials with improved optical circular dichroism.[2] This interest stems from a wide range of applications in biology and physics, including the structural determination of proteins and DNA [3] and the pursuit of negative refraction.[4] Surfaceplasmon-mediated circular dichroism (SP-CD) in solution has been explored to date using small spherical metal particles, invariably resulting in moderate signals over a narrow spectral range. [5][6][7][8][9][10] In contrast, we present herein a novel class of metamaterial consisting of gold nanorods (NRs) organized in three-dimensional (3D) chiral structures and yielding a record circular dichroism anisotropy factor for metal nanoparticles (> 0.02) across visible and near-infrared (Vis-NIR) wavelengths (600-900 nm). The fabrication process can be easily upscaled, as it involves the self-assembly of gold nanorods on a fiber backbone with chiral morphology. Our measurements are fully supported by theoretical modeling based on coupled dipoles, unraveling the key role of gold nanorods in the chiroptical response.Three major strategies have been considered for the generation of SP-CD responses: synthesis of metal clusters with an intrinsically chiral surface, [11,12] adsorption of chiral molecules onto achiral metal nanoparticles, [5,13] and organization of nanoparticles into three-dimensional chiral arrangements.[5-10] Herein, we present a record level of optical activity with a chiral assembly of NRs, which we interpret as SP-CD. Known as plasmonic nanoantennas, [14] these particles are characterized by the resonant collective interaction of their conduction electrons with light in the form of both scattering and absorption, with resonance frequencies that can be tuned across the Vis-NIR spectrum by simply changing the aspect ratio of the nanocrystals.[15] Moreover, the LSPR of NRs is very sensitive to the presence and relative orientation of neighboring particles.[16] These two properties combined make NRs promising building blocks for intense and tunable SP-CD. We have designed a new nanocomposite using a self-assembly strategy [17] with NRs adsorbed onto a scaffold of supramolecular fibers with chiral morphology through specific non-covalent interactions.NRs with an average length of 45 nm and average width of 17 nm were prepared by a seeding growth method, [18] and subsequently coated with the amphiphilic polymer poly(vinylpyrrolidone) (PVP) in ethanol. [19] Fibers having a chiral morphology were obtained by adding water to a DMF/ ethanol solution of anthraquinone-based oxalamide 1 (Figure 1 a), [20] forming a fluid dispersion (see the Supporting Information for experimental details). In Figure 1 b,c, we present scanning electron microscopy (SEM) images of twisted fibers with right-(P) and left-handedness (M), corresponding to (R)-1 and (S)-1, respectively, with widths in the hundred-nanometer range and lengths of several micrometers.For the preparation o...