Abstract

Artifi cially 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. VLADIMIR M. SHALAEV School of Electrical and Computer Engineering and Birck Nanotechnology Center, Purdue University, West Lafayette, Indiana 47907, USA. e-mail: shalaev@purdue.edu Light is the ultimate means of sending information to and from the interior structure of materials -it packages data in a signal of zero mass and unmatched speed. However, light is, in a sense, 'one-handed' when interacting with atoms of conventional materials. Th is is because from the two fi eld components of light -electric and magnetic -only the electric 'hand' effi ciently probes the atoms of a material, whereas the magnetic component remains relatively unused because the interaction of atoms with the magnetic-fi eld component of light is normally weak. Metamaterials, that is, artifi cial materials with rationally designed properties, can allow both fi eld components of light to be coupled 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. Th e 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 fi ne structures that are much smaller than the wavelength of light. Other exciting applications of metamaterials include antennae with superior properties, optical nanolithography and nanocircuits, and 'metacoatings' that can make objects invisible. Th e 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 that are not found 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? Th e notion of metamaterials, which includes a wide range of engineered materials with pre-designed properties, has been used, for example, in the microwave community for some time. Th e idea of metamaterials has been quickly adopted in optics research, thanks to rapidly developing nanofabrication and subwavelength imaging techniques. Metamaterials are expected to open a new gateway to unprecedented electromagnetic properties and functionality unattainable from naturally occurring materials. Th e structural units of metamaterials can be tailored in shape and size. Th eir composition and morphology can be artifi cially tuned, and inclusions can be designed and placed at desired locations to achieve new functionality. One of the most exciting opportunities for metamaterials is the development of negative-index materials (NIMs). Th ese NIMs bring the concept of refractive index into a new domain of exploration and thus promise to create entirely new prospects for manipulating light, with revolutionary impacts on present-day optical technologies. Th e arrival of NIMs provides a rather unique opportunity for researchers to reconsider and possibly even revise the interpretation of very basic laws. Th e notion of a negative refractive index is one such case. Th is is because the index of refraction enters into the basic formulae for optics. As a result, bringing the refractive index into a new domain of negative values has truly excited the imagination of researchers worldwide. Th e refractive index is a complex number n = n´ + in˝, where the imaginary part n˝ characterizes light extinction (losses). Th e real part of the refractive index n´ gives the factor by which the phase velocity of light is decreased in a material as compared with vacuum. NIMs have a negative refractive index, so the phase velocity is directed against the fl ow of energy in a NIM. Th is is highly unusual from the standpoint of 'conventional' optics. Also, at an interface between a positive-and a negative-index material, the refracted light is bent in the 'wrong' way with respect to the normal. Furthermore, the vectors E, H and k form a left -handed system (hence NIMs are also called 'left -handed' materials). Despite all these unusual properties, it is probably not that surprising to learn that a few scientifi c giants considered phenomena related to NIMs quite some time ago. Th eir studies were perhaps so early that they could not be fully appreciated by their contemporaries. Negative phase velocity and its consequences were discussed in works by Sir Arthur Schuster 1 and H. . Veselago has provided the modern prescription of 'negative permittivity/negative permeability' for negative refraction, and he carried through the ramifi cations of this to many optical phenomena. Th e recent boom in NIMs was inspired by Sir John Pendry, who made a number of critical contributions to the fi eld including his famous prediction of the NIM-based superlens with resolution beyond the diff raction limit 6 (see Box 1). No naturally existing NIM has yet been discovered for the optical range of frequencies, where the properties of 'two-handed' light could be particularly spectacular. Th erefore, it is necessary to turn to man-made, artifi cial materials that are composed in such a way that the eff ective refractive index is less than zero, n´e ff < 0.