Progress in Optics (eBook)

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2014 | 1. Auflage
408 Seiten
Elsevier Science (Verlag)
978-0-12-800442-5 (ISBN)

Infos

In the 50 years since the first volume of Progress in Optics was published, optics has become one of the most dynamic fields of science. The volumes in this series that have appeared up to now contain more than 300 review articles by distinguished research workers, which have become permanent records for many important developments, helping optical scientists and optical engineers stay abreast of their fields. - Comprehensive, in-depth reviews - Edited by the leading authority in the field

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Chapter 1

Active Optical Metamaterials

Sebastian Wuestner and Ortwin Hess,    The Blackett Laboratory, Department of Physics, Imperial College London, South Kensington Campus, London SW7 2AZ, United Kingdom,    Email: [e-mail]

Abstract

Metamaterials are composite media that can be engineered to exhibit unique electromagnetic properties. Made up from subwavelength building blocks (most often based on metals), these metamaterials allow for extreme control over optical fields, enabling effects such as negative refraction to be realized. While metallic structural features provide the necessary strong resonant interaction with light, they also give rise to dissipative losses, which can impact negatively on a metamaterial’s performance. The incorporation of optical gain into metamaterials has recently been proposed as a way to counteract those losses.

In this work the complex physics arising from the nonlinear dynamic interaction of optical gain and resonant electromagnetic modes in nanoplasmonic metamaterials are discussed. This is achieved by numerically solving Maxwell’s equations within a finite-difference time-domain (FDTD) framework together with microscopic Maxwell-Bloch equations to describe the (quantum) gain material.

Two relevant topics are considered: First, loss compensation in the negative refractive index regime of a double-layer nano-fishnet metamaterial and second, above-threshold lasing dynamics arising from the plasmonic feedback in the metamaterial. It is shown that loss compensation via optical gain is fundamentally possible and that, in addition, it constitutes a practical means to overcome dissipative losses. Compensation of losses is observed in combination with a negative refractive index, as a result disproving theoretical claims that this should be prohibited by energy conservation arguments. Beyond the regime of amplification, i.e., when the supplied gain exceeds both dissipative losses and radiative outcoupling, lasing instabilities occur. Nonlinear mode competition is observed in this regime and it is found that, despite the presence of a dark plasmonic mode that competes for the gain, sole bright emission can be achieved by appropriate frequency tuning or pump polarization tuning.

Achieving practical, low-loss designs has been a long-standing aim of metamaterials research. By demonstrating the potential to fully compensate losses, the performance of a whole host of existing metamaterial designs could be enhanced through the incorporation of optical gain. Furthermore, gain-enhanced plasmonic metamaterials inherently entail the possibility for coherent feedback through plasmonic modes allowing for above-threshold field dynamics, which may ultimately result in the realization of coherent emission of light from metamaterials and its control through the metamaterials’ unique optical properties.

Keywords

Metamaterials with quantum gain; Active nanoplasmonics; Gain-enhancement; Loss-compensation; Nano-lasing; Maxwell-Bloch Langevin theory

1 Introduction

At the turn of the century, a new field of research emerged at the crossing between physics, electrical engineering, and materials science. The study of metamaterials addresses the rational design and arrangement of a material’s building blocks to attain physical properties that may go significantly beyond those of its original constituent materials. Most often, the concept of metamaterials is associated with electromagnetic wave propagation, where the engineering of resonant subwavelength structures allows for the precise control of effective wave properties. It is this advanced functionality that enables the realization of unique wave phenomena, such as the focusing of light below the diffraction limit or electromagnetic cloaking of objects—concepts that have captured the imagination of researchers and the general public alike. Importantly, the element of functionality is an integral part of the metamaterial concept.

Pioneering work by Pendry in 1996 showed that a medium composed of parallel thin metallic wires would allow for a tuning of its electromagnetic response by changes to the diameter of the wires or their spacing (Pendry, Holden, Stewart, & Youngs, 1996). Functionality in this wire-mesh (WM) medium is thus derived from structural parameters, making it one of the first metamaterials conceived. Yet this work is closely connected with earlier research in electrical engineering in the 1950s on “artificial dielectrics,” where the engineering of effective electromagnetic properties at microwaves and longer wavelengths enabled the realization of lightweight, metallic delay lenses (Milonni, 2005). It was also realized at the time that an effective diamagnetic response could arise from the interaction of electromagnetic waves with the metallic subwavelength building blocks of these lenses. In a breakthrough study in 1999, Pendry demonstrated that the design flexibility of metamaterials enables the realization of structures that exhibit artificial magnetism strong enough to give rise to negative permeability (Pendry, Holden, Robbins, & Stewart, 1999). Original design proposals included the Swiss roll and the split ring resonator (SRR) structure, with cell sizes of the order of 5 mm operating at wavelengths of 10 cm in the microwave regime. The functionality of the metallic SRR building blocks derives from the inductive and capacitive response of free charges and currents in terms of inductive-capacitive resonant loops. The SRR structure in particular proved to be a seminal design in achieving artificial magnetism across the electromagnetic spectrum (Linden et al., 2004; Soukoulis, Linden, & Wegener, 2007).

The unprecedented control over electromagnetic properties provided by metamaterials (Smith, Pendry, & Wiltshire, 2004) revived a reconsideration of initially rather academic studies discussed 30 years earlier by Veselago (1968), Mandelstham, and other renowned scientists (Shalaev, 2007). In those early works, the effects of simultaneous negative permittivity and permeability on wave propagation were investigated and it was predicted that electromagnetic properties would be described by a negative refractive index (NRI). As the refractive index enters most fundamental equations of optics, important physical laws, originally derived for “normal” positive refractive index (PRI) materials, had to be reviewed and fundamental assumptions re-visited. For example, the flow of energy in a NRI material opposes the direction of the phase advance, i.e., the Poynting vector and the wavevector point in opposite directions (Veselago, 1968). This property is also associated with the remarkable fact that Snell’s law predicts negative refraction of a beam of light at the interface between PRI and NRI materials. As a result, the role of convex and concave lenses on the focusing of plane waves interchanges. However, this is not the most exciting property of NRI lenses.

In 2000, it was predicted that an NRI material with permittivity and permeability equal to negative unity would provide the basis for a realization of a “perfect lens” (Pendry, 2000); a lens with imaging resolution that is not diffraction-limited to half the operating wavelength. Indeed, much momentum and excitement in the field of metamaterials originated from this possibility of subwavelength imaging. Yet no less intriguing is the prediction of broadband “stopped light” (the “trapped rainbow”) in a simple waveguide heterostructure exploiting negative phase shifts in structures combining PRI and NRI or plasmonic waveguide materials (Tsakmakidis, Boardman, & Hess, 2007). Another application of metamaterials, which, however, does not necessarily rely on negative refractive index, is the control of the flow of light by gradient index structures in terms of transformation optics (Pendry, Schurig, & Smith, 2006). Transformation optics has, for example, delivered the blueprint for a realization of a metamaterial cloak, which guides light around an electromagnetically forbidden region (Schurig et al., 2006).

In the same year in which the concept of the perfect lens was formulated, Smith and co-workers documented the first demonstration of a metamaterial with negative refractive index at microwave wavelengths (Smith, Padilla, Vier, Nemat-Nasser, & Schultz, 2000). Only a year later, negative refraction at a free-space wavelength of 2.8 cm was observed in a prism constructed from a similar NRI metamaterial (Shelby, Smith, & Schultz, 2001). The metamaterial building blocks in both studies were based on a combination of the WM and SRR media.

An important aim of research in metamaterials in the following years consisted of translating the demonstrated metamaterials functionalities to near-infrared and ultimately optical wavelengths (∼500nm), requiring a miniaturization of the structural elements by more than 4 orders of magnitude. Realizing these size reductions proved technologically challenging and necessitated the use of advanced fabrication techniques. The resulting metallic nanostructures support surface plasmon (SP) resonances, collective oscillations of the free electron gas in the metal excited at the interface to a dielectric by electromagnetic radiation (Barnes, Dereux, & Ebbesen, 2003). These SP resonances strongly bind light at optical wavelengths on and to the nanometer scale. As the electromagnetic response of...

Produkt-Details

• Erscheint lt. Verlag: 8.5.2014
• Mitarbeit: Herausgeber (Serie): Emil Wolf
• Sprache: englisch
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Atom- / Kern- / Molekularphysik
• Themenwelt: Naturwissenschaften ► Physik / Astronomie ► Optik
• Themenwelt: Technik
• ISBN-10: 0-12-800442-8 / 0128004428
• ISBN-13: 978-0-12-800442-5 / 9780128004425