Plasmonics has emerged as an important research field in nanoscience and nanotechnology. Recently, significant attention has been devoted to the observation and the understanding of nonlinear optical processes in plasmonic nanostructures, giving rise to the new research field called nonlinear plasmonics. This review provides a comprehensive insight into the physical mechanisms of one of these nonlinear optical processes, namely, second harmonic generation (SHG), with an emphasis on the main differences with the linear response of plasmonic nanostructures. The main applications, ranging from the nonlinear optical characterization of nanostructure shapes to the optimization of laser beams at the nanoscale, are summarized and discussed. Future directions and developments, made possible by the unique combination of SHG surface sensitivity and field enhancements associated with surface plasmon resonances, are also addressed.

In this review we describe fundamentals of scanning near-field optical microscopy with aperture probes. After the discussion of instrumentation and probe fabrication, aspects of light propagation in metal-coated, tapered optical fibers are considered. This includes transmission properties and field distributions in the vicinity of subwavelength apertures. Furthermore, the near-field optical image formation mechanism is analyzed with special emphasis on potential sources of artifacts. To underline the prospects of the technique, selected applications including amplitude and phase contrast imaging, fluorescence imaging, and Raman spectroscopy, as well as near-field optical desorption, are presented. These examples demonstrate that scanning near-field optical microscopy is no longer an exotic method but has matured into a valuable tool.

We investigate numerically the spectrum of plasmon resonances for metallic nanowires with a nonregular cross section, in the 20-50 nm range. We first consider the resonance spectra corresponding to nanowires whose cross sections form different simplexes. The number of resonances strongly increases when the section symmetry decreases: A cylindrical wire exhibits one resonance, whereas we observe more than five distinct resonances for a triangular particle. The spectral range covered by these different resonances becomes very large, giving to the particle-specific distinct colors. At the resonance, dramatic field enhancement is observed at the vicinity of nonregular particles, where the field amplitude can reach several hundred times that of the illumination field. This near-field enhancement corresponds to surface-enhanced Raman scattering ͑SERS͒ enhancement locally in excess of 10 12 . The distance dependence of this enhancement is investigated and we show that it depends on the plasmon resonance excited in the particle, i.e., on the illumination wavelength. The average Raman enhancement for molecules distributed on the entire particle surface is also computed and discussed in the context of experiments in which large numbers of molecules are used.

Abstract:The optical properties of plasmonic dipole and bowtie nanoantennas are investigated in detail using the Green's tensor technique. The influence of the geometrical parameters (antenna length, gap dimension and bow angle) on the antenna field enhancement and spectral response is discussed. Dipole and bowtie antennas confine the field in a volume well below the diffraction limit, defined by the gap dimensions. The dipole antenna produces a stronger field enhancement than the bowtie antenna for all investigated antenna geometries. This enhancement can reach three orders of magnitude for the smallest examined gap. Whereas the dipole antenna is monomode in the considered spectral range, the bowtie antenna exhibits multiple resonances. Furthermore, the sensitivity of the antennas to index changes of the environment and of the substrate is investigated in detail for biosensing applications; the bowtie antennas show slightly higher sensitivity than the dipole antenna.

We present a new theoretical and numerical framework for the study of the optical properties of micrometric and nanometric three-dimensional structures of arbitrary shape. We show that the field distribution induced inside and outside such a structure by different external monochromatic sources can be obtained from a unique generalized field propagator expressed in direct space. An application of the method to the confinement of optical fields due to the scattering of subwavelength objects is presented. 02.30.Tb, 02.60.Nm, 61.16.Ch Over the past few years, considerable efforts have been devoted to the understanding of the response properties of micrometric and nanometric structures isolated in gas phase [1] or integrated on a surface [2]. Recent continuous progress in scanning near-field optical microscopy (SNOM) [3][4][5][6] has strongly enhanced our insight into the field distributions in the vicinity of such subwavelength structures. From a theoretical point of view, dealing with low-symmetry, three-dimensional (3D) systems renders the intricate problem related to the electromagnetic boundary conditions at the material interfaces intractable. Therefore, most of the numerical approaches based on the matching of these boundary conditions encounter difficulties when applied to geometrically complex mesoscopic and subwavelength systems made of realistic materials. This fact was demonstrated by the theoretical challenge raised by the development of SNOM.In order to circumvent this obstacle inherent to the matching of electromagnetic boundary conditions, we present in this Letter a new theoretical framework for a large class of problems dealing with 3D objects of arbitrary shape and dielectric functions. More precisely, we show that the entire field distribution induced by different sources, inside and outside a 3D structure, can be derived from a unique generalized field propagator expressed in direct space. This approach is based on Green's dyadic technique. Although the power of this technique has long been recognized, its use was impeded by the difficult construction of Green's dyadics associated with complex geometries. In this Letter, we point out that electromagnetic scattering theory gains much efficiency by adopting certain procedures from quantum scattering theory. Particularly, the introduction of a dyadic Dyson's equation enables the straightforward construction of Green's dyadics associated with arbitrarily complex geometries.Let us consider a nonmagnetic scattering system described by a dielectric tensor´͑r, v͒ ´r ͑v͒ 1´s͑r, v͒, embedded in an infinite homogeneous reference mediuḿ r ͑v͒. This scattering system must not necessarily be a single region, but can be formed by disconnected parts embedded in the reference system, with the tensor´s͑r, v͒ vanishing outside of the scattering system.When an incident field E 0 ͑r͒ (a monochromatic field with the usual exp͓2ivt͔ time dependence is assumed throughout this Letter, but the method is able to handle arbitrary incident waves) impinges on that system...

Abstract:We investigate the plasmon resonances of interacting silver nanowires with a 50 nm diameter. Both non-touching and intersecting configurations are investigated. While individual cylinders exhibit a single plasmon resonance, we observe much more complex spectra of resonances for interacting structures. The number and magnitude of the different resonances depend on the illumination direction and on the distance between the particles. For very small separations, we observe a dramatic field enhancement between the particles, where the electric field amplitude reaches a hundredfold of the illumination. A similar enhancement is observed in the grooves created in slightly intersecting particles. The topology of these different resonances is related to the induced polarization charges. The implication of these results to surface enhanced Raman scattering (SERS) are discussed. Spectrosc. 31, 625-631 (2000). 11. T. J. Silva and S. Schultz, "A scanning near-field optical microscope for the imaging of magnetic domains in reflection," Rev. Sci. Inst. 67, 715-725 (1996). 2590-2593 (1999).

We present a technique for the computation of the Green's tensor in three-dimensional stratified media composed of an arbitrary number of layers with different permittivities and permeabilities ͑including metals with a complex permittivity͒. The practical implementation of this technique is discussed in detail. In particular, we show how to efficiently handle the singularities occurring in Sommerfeld integrals, by deforming the integration path in the complex plane. Examples assess the accuracy of this approach and illustrate the physical properties of the Green's tensor, which represents the field radiated by three orthogonal dipoles embedded in the multilayered medium.

Among the most popular approaches used for simulating plasmonic systems, the discrete dipole approximation suffers from poorly scaling volume discretization and limited near-field accuracy. We demonstrate that transformation to a surface integral formulation improves scalability and convergence and provides a flexible geometric approximation allowing, e.g., to investigate the influence of fabrication accuracy. The occurring integrals can be solved quasi-analytically, permitting even rapidly changing fields to be determined arbitrarily close to a scatterer. This insight into the extreme near-field behavior is useful for modeling closely packed particle ensembles and to study "hot spots" in plasmonic nanostructures used for plasmon-enhanced Raman scattering.