Fundamentals of Plasmonics.- Electromagnetics of Metals.- Surface Plasmon Polaritons at Metal / Insulator Interfaces.- Excitation of Surface Plasmon Polaritons at Planar Interfaces.- Imaging Surface Plasmon Polariton Propagation.- Localized Surface Plasmons.- Electromagnetic Surface Modes at Low Frequencies.- Applications.- Plasmon Waveguides.- Transmission of Radiation Through Apertures and Films.- Enhancement of Emissive Processes and Nonlinearities.- Spectroscopy and Sensing.- Metamaterials and Imaging with Surface Plasmon Polaritons.- Concluding Remarks.

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Plasmonics: Fundamentals and Applications

Plasmons in strongly coupled metallic nanostructures.

Artificially engineered metamaterials are now demonstrating unprecedented electromagnetic properties that cannot be obtained with naturally occurring materials. In particular, they provide a route to creating materials that possess a negative refractive index and offer exciting new prospects for manipulating light. This review describes the recent progress made in creating nanostructured metamaterials with a negative index at optical wavelengths, and discusses some of the devices that could result from these new materials.

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Optical negative-index metamaterials

Coinage metals, such as Au, Ag, and Cu, have been important materials throughout history.1 While in ancient cultures they were admired primarily for their ability to reflect light, their applications have become far more sophisticated with our increased understanding and control of the atomic world. Today, these metals are widely used in electronics, catalysis, and as structural materials, but when they are fashioned into structures with nanometer-sized dimensions, they also become enablers for a completely different set of applications that involve light. These new applications go far beyond merely reflecting light, and have renewed our interest in maneuvering the interactions between metals and light in a field known as plasmonics.2–6

In plasmonics, the metal nanostructures can serve as antennas to convert light into localized electric fields (E-fields) or as waveguides to route light to desired locations with nanometer precision. These applications are made possible through a strong interaction between incident light and free electrons in the nanostructures. With a tight control over the nanostructures in terms of size and shape, light can be effectively manipulated and controlled with unprecedented accuracy.3,7 While many new technologies stand to be realized from plasmonics, with notable examples including superlenses,8 invisible cloaks,9 and quantum computing,10,11 conventional technologies like microprocessors and photovoltaic devices could also be made significantly faster and more efficient with the integration of plasmonic nanostructures.12–15 Of the metals, Ag has probably played the most important role in the development of plasmonics, and its unique properties make it well-suited for most of the next-generation plasmonic technologies.16–18

1.1. What is Plasmonics?
Plasmonics is related to the localization, guiding, and manipulation of electromagnetic waves beyond the diffraction limit and down to the nanometer length scale.4,6 The key component of plasmonics is a metal, because it supports surface plasmon polariton modes (indicated as surface plasmons or SPs throughout this review), which are electromagnetic waves coupled to the collective oscillations of free electrons in the metal.

While there are a rich variety of plasmonic metal nanostructures, they can be differentiated based on the plasmonic modes they support: localized surface plasmons (LSPs) or propagating surface plasmons (PSPs).5,19 In LSPs, the time-varying electric field associated with the light (Eo) exerts a force on the gas of negatively charged electrons in the conduction band of the metal and drives them to oscillate collectively. At a certain excitation frequency (w), this oscillation will be in resonance with the incident light, resulting in a strong oscillation of the surface electrons, commonly known as a localized surface plasmon resonance (LSPR) mode.20 This phenomenon is illustrated in Figure 1A. Structures that support LSPRs experience a uniform Eo when excited by light as their dimensions are much smaller than the wavelength of the light.



Figure 1

Schematic illustration of the two types of plasmonic nanostructures discussed in this article as excited by the electric field (Eo) of incident light with wavevector (k). In (A) the nanostructure is smaller than the wavelength of light and the free electrons ...



In contrast, PSPs are supported by structures that have at least one dimension that approaches the excitation wavelength, as shown in Figure 1B.4 In this case, the Eo is not uniform across the structure and other effects must be considered. In such a structure, like a nanowire for example, SPs propagate back and forth between the ends of the structure. This can be described as a Fabry-Perot resonator with resonance condition l=nλsp, where l is the length of the nanowire, n is an integer, and λsp is the wavelength of the PSP mode.21,22 Reflection from the ends of the structure must also be considered, which can change the phase and resonant length. Propagation lengths can be in the tens of micrometers (for nanowires) and the PSP waves can be manipulated by controlling the geometrical parameters of the structure.23

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Controlling the synthesis and assembly of silver nanostructures for plasmonic applications

In the Raman effect, incident light is inelastically scattered from a sample and shifted in frequency by the energy of its characteristic molecular vibrations. Since its discovery in 1927, the effect has attracted attention from a basic research point of view as well as a powerful spectroscopic technique with many practical applications. The advent of laser light sources with monochromatic photons at high flux densities was a milestone in the history of Raman spectroscopy and resulted in dramatically improved scattering signals (for a general overview of modern Raman spectroscopy, see refs 1-5). In addition to this so-called spontaneous or incoherent Raman scattering, the development of lasers also opened the field of stimulated or coherent Raman spectroscopies, in which molecular vibrations are coherently excited. Whereas the intensity of spontaneous Raman scattering depends linearly on the number of probed molecules, the coherent Raman signal is proportional to the square of this number (for an overview, see refs 6 and 7). Coherent Raman techniques can provide interesting new opportunities such as vibrational imaging of biological samples,8 but they have not yet advanced the field of ultrasensitive trace detection. Therefore, in the following article, we shall focus on the spontaneous Raman effect, in the following simply called Raman scattering. Today, laser photons over a wide range of frequencies from the near-ultraviolet to the near-infrared region are used in Raman scattering studies, allowing selection of optimum excitation conditions for each sample. By choosing wavelengths which excite appropriate electronic transitions, resonance Raman studies of selected components of a sample or parts of a molecule can be performed.9 In the past few years, the range of excitation wavelengths has been extended to the near-infrared (NIR) region, in which background fluorescence is reduced and photoinduced degradation from the sample is diminished. High-intensity NIR diode lasers are easily available, making this region attractive for compact, low cost Raman instrumentation. Further, the development of low noise, high quantum efficiency multichannel detectors (chargecoupled device (CCD) arrays), combined with highthroughput single-stage spectrographs used in combination with holographic laser rejection filters, has led to high-sensitivity Raman spectrometers (for an overview on state-of-the-art NIR Raman systems, see ref 10). As we shall show in section 2, the nearinfrared region also has special importance for ultrasensitive Raman spectroscopy at the singlemolecule level. As with optical spectroscopy, the Raman effect can be applied noninvasively under ambient conditions in almost every environment. Measuring a Raman spectrum does not require special sample preparation techniques, in contrast with infrared absorption spectroscopy. Optical fiber probes for bringing excitation laser light to the sample and transporting scattered light to the spectrograph enable remote detection of Raman signals. Furthermore, the spatial and temporal resolution of Raman scattering are determined by the spot size and pulse length, respectively, of the excitation laser. By using a confocal microscope, Raman signals from femtoliter volumes (∼1 μm3) can by observed, enabling spatially resolved measurements in chromosomes and cells.11 Techniques such as multichannel Hadamard transform Raman microscopy12,13 or confocal scanning Fourier transform Raman microscopy14 allow generation of high-resolution Raman images of a sample. Recently, Raman spectroscopy was performed using near-field optical microscopy.15-17 Such techniques overcome the diffraction limit and allow volumes significantly smaller than the cube of the wavelength to be investigated. In the time domain, Raman spectra can be measured on the picosecond time scale, providing information on short-lived species such as excited 2957 Chem. Rev. 1999, 99, 2957−2975

Ultrasensitive Chemical Analysis by Raman Spectroscopy

We present a detailed theoretical study of the dielectric and magnetic response of composites containing elongated conducting inclusions---sticks. These composites are widely used as engineering materials. They can be also considered as a model to describe many processes occurring in nature, e.g., dielectric enhancement in grain-saturated porous rocks. An approach is proposed that is based on the idea of a scale-dependent local dielectric constant. We develop an effective-medium approximation and derive an equation to calculate an effective dielectric constant of the composites in the quasistatic case and for the high frequency when there is a strong skin effect in the conducting sticks. Our theory predicts very large values of the effective dielectric constant in a wide range of the stick concentration. We find that the dielectric constant can exhibit various dispersive behaviors. It can have relaxation behavior, power-law scaling behavior, or resonance dependence on the frequency. The resonance dependence occurs when the skin effect is strong and wavelength is comparable to the stick length. Then the real part of the dielectric constant has negative values in some frequency ranges. The possibility of a wave localization is discussed in that case. We consider effective magnetic properties of the conducting stick composites. We propose that the composites with nonmagnetic components will have a giant paramagnetic response as a result of a collective interaction of the sticks with an external magnetic field. \textcopyright{} 1996 The American Physical Society.

Electromagnetic properties of composites containing elongated conducting inclusions

Light is in a sense "one-handed" when interacting with atoms of conventional materials. This is because out of the two field components of light, electric and magnetic, only the electric "hand" efficiently probes the atoms of a material, whereas the magnetic component remains relatively unused because the interaction of atoms with the magnetic field component of light is normally weak. Metamaterials, i.e. artificial materials with rationally designed properties, can enable the coupling of both of the field components of light to meta-atoms, enabling entirely new optical properties and exciting applications with such "two-handed" light. Among the fascinating properties is a negative refractive index. The refractive index is one of the most fundamental characteristics of light propagation in materials. Metamaterials with negative refraction may lead to the development of a superlens capable of imaging objects and their fine structures that are much smaller than the wavelength of light. Other exciting applications of metamaterials include novel antennae with superior properties, optical nano-lithography and nano-circuits, and "meta-coatings" that can make objects invisible. The word "meta" means "beyond" in Greek, and in this sense the name "metamaterials" refers to "beyond conventional materials." Metamaterials are typically man-made and have properties not available in nature. What is so magical about this simple merging of "meta" and "materials" that has attracted so much attention from researchers and has resulted in exponential growth in the number of publications in this area? The answer you can find in this book.

Electrodynamics of Metamaterials

We show that tapered arrays of thin metallic wires can manipulate electromagnetic fields on the subwavelength spatial scale. Two types of nanoscale imaging applications using terahertz and midinfrared waves are enabled: image magnification and radiation focusing. First, the tapered wire array acts as a multipixel TEM endoscope by capturing an electromagnetic field profile created by deeply subwavelength objects at the endoscope's tip and magnifying it for observation. Second, the image of a large mask at the endoscope's base is projected onto a much smaller image at the tip.

https://arxiv.org/pdf/physics/0702049.pdf

Guiding, focusing, and sensing on the subwavelength scale using metallic wire arrays.

We report a numerical investigation of surface plasmon (SP) propagation in ordered and disordered linear chains of metal nanospheres. In our simulations, SPs are excited at one end of a chain by a near-field tip. We then find numerically the SP amplitude as a function of propagation distance. Two types of SPs are discovered. The first SP, which we call the ordinary or quasistatic, is mediated by short-range, near-field electromagnetic interaction in the chain. This excitation is strongly affected by Ohmic losses in the metal and by disorder in the chain. These two effects result in spatial decay of the quasistatic SP by means of absorptive and radiative losses, respectively. The second SP is mediated by longer range, far-field interaction of nanospheres. We refer to this SP as the extraordinary or nonquasistatic. The nonquasistatic SP cannot be effectively excited by a near-field probe due to the small integral weight of the associated spectral line. Because of that, at small propagation distances, this SP is dominated by the quasistatic SP. However, the nonquasistatic SP is affected by Ohmic and radiative losses to a much smaller extent than the quasistatic one. Because of that, the nonquasistatic SP becomes dominant sufficiently far from the exciting tip and can propagate with little further losses of energy to remarkable distances. The unique physical properties of the nonquasistatic SP can be utilized in all-optical integrated photonic systems.

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Propagation of surface plasmons in ordered and disordered chains of metal nanospheres

Results are presented of the comprehensive experimental investigation of the dielectric properties of composites filled with conducting fibers results. The results apply to the percolation threshold values and microwave dielectric dispersion of these materials. The fiber-filled composites exhibit a great variety of dielectric dispersion behavior, and offer wide possibilities of tailoring their dielectric dispersion. The experimental data are in good agreement with the recent theoretical approach [see A. N. Lagarkov and A. K. Sarychev, Phys. Rev. B 53, 6318 (1996)] as regards the values of percolation threshold and the shape of dielectric dispersion dependences. This agreement is a validation of the original assumption that the permittivity of effective medium is a scale dependent function in fiber-filled composites.

Andrey K. Sarychev

Papers

Electromagnetic properties of composites containing elongated conducting inclusions

Electrodynamics of Metamaterials

Guiding, focusing, and sensing on the subwavelength scale using metallic wire arrays.

Propagation of surface plasmons in ordered and disordered chains of metal nanospheres

Dielectric properties of fiber-filled composites