Plasmonics and Plasmonic Metamaterials

The following sections are included:

• CONTENTS

• PREFACE

• ACKNOWLEDGMENT FROM I.T.

In this chapter we present a simple and comprehensive explanation of the mechanisms that can so dramatically modify the optical properties of atoms, molecules, or other quantum-size objects placed in the vicinity of metal nanoparticles. We develop a simple model that describes surface plasmon modes supported by the metal nanoparticles and describes them using just three key parameters — effective volume, Q-factor, and radiative decay rate. We subsequently apply this model to the tasks of estimating the enhancement of optical radiation, electroluminescence, and photoluminescence absorbed or emitted by the optically active objects in the presence of an isolated single nanoparticle. Using the example of gold nanospheres embedded in GaN dielectric, we show that enhancement for each case depends strongly on the nanoparticle size enabling optimization for each combination of absorption cross section, original radiative efficiency, and separation between the object and metal sphere. We then expand the model for single metal nanoparticles to coupled metal nanostructures. We show that complex structures can be treated as coupled multipole modes with highest enhancements obtained due to the superposition of these modes mainly in small particles. This model allows for optimization of the structures for the largest possible field enhancements, which depends on the quality factor Q of the metal and can be as high as Q² for two spherical particles. The “hot spot” can occur either in the nano-gaps between the particles or near the smaller particles. We trace the optimal field enhancement mechanism to the fact that the extended dipole modes of larger particles act as the efficient antennas while the modes in the gaps or near the smaller particles act as the compact sub-wavelength cavities. The physically-transparent, comprehensive analytical approach developed in this chapter not only offers a quick route for optimization but also can be conveniently extended to incorporate large numbers of particles in various complex arrangements.

We discuss the wave propagation inside a chiral metallic/plasmonic medium defined by stacking and twisting an anisotropic medium in one direction. An anisotropic transfer matrix method for calculating the band structures and transmission properties of such a medium is described. The optical properties of some dielectric media that have the same kind of chiral structures are also discussed. When a bulk plasma dielectric function is introduced to one of the axis of the anisotropic medium, a W-shape band appears above the plasmonic bandgap. Such a chiral metallic/plasmonic medium supports both positive and negative refraction, and the criteria for achieving a negative refraction is addressed.

Optical properties of plasmonic nanostructures and metamaterials are often accessed by evaluating their interaction with light by means of rigorous numerical methods. Such analysis allows the reliable prediction of any measurable quantity, whereas insights into the physical mechanisms that govern the observable effects require an intense interpretation of these quantities. Therefore, analytical methods are required that simplify the description of plasmonic entities to a certain extent but yet allow the disclosure of their physical peculiarities. We outline in this chapter the basics of such an analytical model which we coined the multipole approach to metamaterials. In this parametric model the elementary constituents that form plasmonic nanostructures are conceptually replaced by coupled dipoles. By describing the evolution of these dipoles in terms of differential equations, we disclose the dynamics of complex nanostructures. Furthermore, by introducing averaged quantities derived from the dipole dynamics, such as an electric and magnetic dipole and an electric quadrupole density, the light propagation in a medium comprising a dense array of these nanostructures is fully accessible. This contribution is written with the intention to familiarize readers with this framework and to allow its application to many related problem that may emerge in the field of plasmonics and metamaterials.

Surface plasmon-polaritons offer useful properties that find applications in a broad range of scientific and engineering fields. However, many applications face practical limitations imposed by the intrinsic energy losses experienced by this wave at optical and near-infrared wavelengths. During the last decade, the topic of surface plasmon-polariton amplification has experienced a tremendous growth as it offers a viable venue to eliminate the wave's losses without compromising other key attributes. This review summarises the major theoretical and experimental progress achieved in the amplification of this wave. It discusses the topics of amplified spontaneous emission, stimulated emission, and lasing within the context of surface plasmon-polaritons.

We review recent work on beam shaping of mid-infrared and far-infrared (terahertz) quantum cascade lasers using plasmonics. Essentials of quantum cascade lasers (QCLs) are discussed; these include the operating principle based on bandstructure engineering, and beam quality problems associated with laser waveguide design. We explain how metal and semiconductor microstructures can effectively tailor the dispersion properties of mid- and far-infrared surface plasmon polaritons, and therefore can be used as important building blocks for optical devices in these frequencies. The physical principles of three structures are discussed: plasmonic Bragg gratings, designer (spoof) surface plasmon polariton structures, and channel polariton structures. We demonstrate the effectiveness of these structures by realizing various functionalities in QCLs, ranging from beam collimation, polarization control, to multibeam emission, and spatial wavelength demultiplexing. Plasmonics offers a monolithic, compact, and low-loss solution to the problem of poor beam quality of QCLs and may have a large impact on applications such as sensing, light detection and ranging (LIDAR), free-space optical communication, and heterodyne detection of chemicals. The plasmonic designs are scalable and applicable to near-infrared active or passive optical devices.

Plasmonics, localizing light to the sub-wavelength dimensions and dramatically enhancing local fields, is enabling new possibilities for realization of advanced biosensors that can detect and analyze small quantities of biomolecules and dangerous pathogens. In this chapter, we will focus on integrated plasmonic systems for ultrasensitive infrared nanospectroscopy and biodetection. Infrared absorption spectroscopy, which directly accesses vibrational fingerprints of the biomolecules/chemicals at mid-IR frequencies, is an important identification and analysis tool. However, small absorption cross sections of the molecules strictly hinder the usage of this technique for identifying small quantities of biological specimen. We will demonstrate diffractively coupled plasmonic nanoantennas enabling ultra-sensitive surface-enhanced spectroscopy with zeptomole level sensitivities. In addition, we will introduce a low-cost fabrication technique for high-throughput fabrication of such engineered infrared plasmonic antenna arrays. Finally, we will describe a novel biosensing system merging nanoplasmonics and nanofluidics to overcome fundamental mass transport limitations imposed by conventional microfluidic approaches. Our detection platform, manipulating light as well as directing flow on the through the nanoholes, enables targeted analyte delivery and dramatically improves sensor response time.

Long-range surface plasmon polariton waveguides form a class of plasmonic waveguide geometries characterized by low propagation loss and weak confinement, relative to other metal-dielectric waveguides. These properties allow for fabrication of a range of plasmonic devices that are compatible with conventional fiber optics while possessing several unique features, including the possibility of strong index modulation, large evanescent-field volumes for optical sensing purposes, and control of waveguide properties by passing electrical current through the waveguide core. Moreover, the low-loss waveguide geometry offers a platform for realizing plasmonic amplification, using conventional organic gain materials like conjugated fluorescent polymers. Here, we describe fabrication and optical characterization of long-range surface plasmon waveguides and devices based on thin metallic stripes or metallic nanowires embedded in a polymer matrix, including thermo-optically controlled interferometric devices, compact extinction modulators with low polarization-dependent loss and optically pumped plasmonic amplifiers.

This chapter introduces a new type of engineered surface — plasmonic crystals with three-dimensional (3D) unit cells — for biosensing. Nanostructured metal films exhibit surface plasmon (SP) resonances with exceptional sensitivities but are limited to bulk refractive index (RI) sensing. Here we describe the biosensing capabilities of two different plasmonic crystal structures that support both surface plasmon polaritons (SPPs) and localized surface plasmons (LSPs): (i) a nano-pyramidal grating and (ii) a 3D nanohole array. These plasmonic crystals not only exhibit bulk RI sensitivities comparable to commercial SP sensors but also have surface sensitivities that can be tuned by manipulating the free-space light excitation angle θ. We show how tailoring the surface sensitivity of the plasmonic crystals optimizes real-time measurements of biomolecular interactions.

Optical negative-index metamaterials (NIMs) have attracted a significant amount of research attention because of their unique properties and important applications including superlenses and cloaking. Recently, after numerous fundamental studies, research interest has turned to the realization of real NIM applications. These applications are strongly limited by the long and fixed working wavelength and the significant losses in NIM designs. In this chapter, we demonstrate our recent progress in the design, fabrication, and characterization of optical NIMs. First, we demonstrate NIMs operating in optical wavelength range. Then we show a method to adjust the refractive index of dielectric layers in a NIM and thereby tune the resonant wavelength of NIM by tens of nanometers. Finally, and most importantly for real-world NIM applications, we introduce active materials into NIMs and show that loss compensation can significantly improve the essential parameters of NIMs. We believe our studies will be very interesting and promising for real NIM applications in the near future.

Nanophotonics is finding myriad applications in information technology, health care, lighting and sensing. Plasmonics has been recently experiencing rapid development due to its myriad applications in nanophotonics. In this chapter, we explore the electrodynamics of plasmonic fields on different structured metallic chips and demonstrate how to manipulate light from nano to micro scale on the structured nanophotonic chips. We investigate on-chip plasmonic metamaterials with novel material responses and functionalities, develop the design methodology for plasmonic chips compatible with the conventional Fourier optical devices operating on diffraction limited plasmonic waves, as well as construct sophisticated chip-scale integration of optical elements with variable scales to achieve deeply subwavelength localization of optical fields.

This chapter concerns modeling, design, and characterization of dielectric-loaded plasmonic waveguide components at telecommunication wavelengths, aimed at establishing a new technology platform for integratable nanophotonic components by combining photonics and electronics on the same chip. The dielectric-loaded surface plasmon-polariton waveguides (DLSPPWs) consist of dielectric ridges with rectangular nano-meter-sized cross-sections, deposited on smooth metal films. The SPPs are strongly confined to the ridges due to the large index contrast between the dielectric ridge region and the surrounding air. Theoretical studies of the DLSPPW structure show that single mode propagation with sub-wavelength confinement and low propagation loss can be achieved by proper design of the waveguide dimensions, which is confirmed by near-field optical imaging of fabricated DLSPPWs, performed with a scanning near-field optical microscope. The performance basic waveguide components along with several different passive wavelength selective components is demonstrated. The coupling between two parallel DLSPPWs is investigated by characterizing directional couplers, where the two waveguides are brought into close proximity of one another by means of S-bends. Periodic modulation of the transmission is achieved by realizing waveguide-ring resonators and wavelength filtering is obtained by realizing Bragg gratings. In addition an approach for designing components capable of physically separating two signals of different wavelengths is introduced.

Field enhancement from surface plasmon structures presents new opportunities for optical manipulation and surface enhanced Raman spectroscopy (SERS). We demonstrate the propulsion of gold nanoparticles using surface plasmon polaritons. SPPs are excited on a thin gold film. The resultant evanescent field draws nanoparticles toward the film, where they are propelled along by the optical scattering force. We describe our work on a novel SERS substrate consisting of a metal nanoparticle array separated from a gold film by a thin silicon dioxide spacer. We show that the double plasmon resonances of these structures enable strong field enhancement at both pump and Stokes frequencies.

In the last years light scattering by nanostructures is of interest in different areas of science and technology. Analysis of light scattered by nanostructures is an effective tool for a better understanding of their properties. In this work the Discrete Sources Method (DSM) is applied to model light scattering by nanoparticles on a surface. One of attractive features of the DSM is an ability to account for all the features of the modeled system, such as complex refractive index with frequency dispersion of particles and a substrate, scattering interaction of particle and an interface. To demonstrate the variety of possible applications for the DSM, we concentrated on two practical applications. First is light scattering by a nanorod on a surface, which requires the use of a general 3D version of the DSM. The second case discussed in this chapter is light scattering by a nanoshell, which allows the accounting for the axial symmetry of the problem and essential reduction of calculation time. In both cases light scattering characteristics and their dependence on nanostructure characteristics like size, symmetry, incident angle, particle orientation, refractive index and wavelength are analyzed and discussed.

First, the main computational techniques for solving electromagnetic problems with focus on plasmonics, antennas, and waveguides are reviewed. Special attention is paid to special symmetries, such as symmetry with respect to one or several planes, cylindrical symmetry, rotational symmetry, periodic symmetry in one or more directions, and combinations of these symmetries. Then, tricky problems, such as a metallic tip — excited by a fundamental wire mode — and a finite chain of metallic spheres, embedded in a dielectric cylinder are investigated using the finite element method (FEM) and the multiple multipole program (MMP) in order to illustrate the procedures.

The following sections are included:

• Index