ScienceWeek

MATERIALS SCIENCE: ON METAMATERIALS

The following points are made by D.R. Smith et al (Science 2004 305:788):

1) Consider light passing through a plate of glass. We know that light is an electromagnetic wave, consisting of oscillating electric and magnetic fields, and characterized by a wavelength, L. Because visible light has a wavelength that is hundreds of times larger than the atoms of which the glass is composed, the atomic details lose importance in describing how the glass interacts with light. In practice, we can average over the atomic scale, conceptually replacing the otherwise inhomogeneous medium by a homogeneous material characterized by just two macroscopic electromagnetic parameters: the electric permittivity, E, and the magnetic permeability, M.

2) From the electromagnetic point of view, the wavelength, L, determines whether a collection of atoms or other objects can be considered a material. The electromagnetic parameters E and M need not arise strictly from the response of atoms or molecules: Any collection of objects whose size and spacing are much smaller than L can be described by an E and M. Here, the values of E and M are determined by the scattering properties of the structured objects. Although such an inhomogeneous collection may not satisfy our intuitive definition of a material, an electromagnetic wave passing through the structure cannot tell the difference. From the electromagnetic point of view, we have created an artificial material, or "metamaterial".

3) The engineered response of metamaterials has had a dramatic impact on the physics, optics, and engineering communities, because metamaterials can offer electromagnetic properties that are difficult or impossible to achieve with conventional, naturally occurring materials. The advent of metamaterials has yielded new opportunities to realize physical phenomena that were previously only theoretical exercises.

4) In 1999, several artificial materials were introduced, based on conducting elements designed to provide a magnetic response at microwave and lower frequencies (1). These nonmagnetic structures consisted of arrays of wire loops in which an external applied magnetic field could induce a current, thus producing an effective magnetic response.

5) The possibility of magnetism without inherently magnetic materials turns out to be a natural match for magnetic resonance imaging (MRI), which we use as an example of a potential application area for metamaterials. In an MRI machine there are two distinct magnetic fields. Large quasi-static fields, between 0.2 and 3 tesla in commercial machines, cause the nuclear spins in a patient's body to align. The spins are resonant at the local Larmor frequency, typically between 8.5 and 128 MHz, so that a second magnetic field in the form of a radio frequency (RF) pulse will excite them, causing them to precess about the main field. Images are reconstructed by observing the time-dependent signal resulting from the precession of the spins. Although the resolution of an MRI machine is obtained through the quasistatic fields, precise control of the RF field is also vital to the efficient and accurate operation of the machine.

6) Any material destined for use in the MRI environment must not perturb the quasi-static magnetic field pattern, thus excluding the use of all conventional magnetic materials. However magnetic metamaterials that respond to time-varying fields but not to static fields can be used to alter and focus the RF fields without interfering with the quasi-static field pattern.

7) In summary: Artificially constructed metamaterials have become of considerable interest because these materials can exhibit electromagnetic characteristics unlike those of any conventional materials. Artificial magnetism and negative refractive index are two specific types of behavior that have been demonstrated over the past few years, illustrating the new physics and new applications possible when we expand our view as to what constitutes a material.(2-5)

References (abridged):

1. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, IEEE Trans. Microwave Theory Tech. 47, 2075 (1999)

2. M. C. K. Wiltshire et al., Science 291, 849 (2001)

3. M. C. K. Wiltshire, J. V. Hajnal, J. B. Pendry, D. J. Edwards, C. J. Stevens, Opt. Express 11, 709 (2003)

4. M. C. K. Wiltshire et al., Proc. Int. Soc. Mag. Reson. Med. 11, 713 (2003)

5. T. J. Yen et al., Science 303, 1494 (2004)

Science http://www.sciencemag.org

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Related Material:

OPTICS: ON NEGATIVE REFRACTION

The following points are made by J.B. Pendry and D.R. Smith (Physics Today 2004 June):

1) Victor Veselago, in a paper(1) published in 1968, pondered the consequences for electromagnetic waves interacting with a hypothetical material for which both the electric permittivity (e) and the magnetic permeability (m) were simultaneously negative. Because no naturally occurring material or compound has ever been demonstrated with negative (e) and (m), Veselago wondered whether this apparent asymmetry in material properties was just happenstance or perhaps had a more fundamental origin. He concluded that not only should such materials be possible, but if ever found, they would exhibit remarkable properties unlike those of any known materials and would give a twist to virtually all electromagnetic phenomena. Foremost among these properties is a negative index of refraction.

2) Veselago always referred to the materials as "left handed", because the wave vector is antiparallel to the usual right-handed cross product of the electric and magnetic fields. The authors prefer the negative-index description. The names mean the same thing, but the authors suggest their description appeals more to everyday intuition and is less likely to be confused with chirality, an entirely different phenomenon.

3) Why are there no materials with negative (e) and (m)? One first needs to understand what it means to have a negative (e) or (m) and how negative values occur in materials. The Drude-Lorentz model of a material is a good starting point, because it conceptually replaces the atoms and molecules of a real material by a set of harmonically bound electron oscillators resonant at some frequency (F). At frequencies far below (F), an applied electric field displaces the electrons from the positive cores and induces a polarization in the same direction as the applied field. At frequencies near resonance, the induced polarization becomes very large, as is typical in resonance phenomena; the large response represents accumulation of energy over many cycles, such that a considerable amount of energy is stored in the resonator (in this case, the medium) relative to the driving field.

4) So large is this stored energy that even changing the sign of the applied electric field has little effect on the polarization near resonance. That is, as the frequency of the driving electric field is swept through the resonance, the polarization flips from in-phase to out-of-phase with the driving field, and the material exhibits a negative response. If instead of electrons the material response were due to harmonically bound magnetic moments, then a negative magnetic response would exist.

5) Although somewhat less common than positive materials, negative materials are nevertheless easy to find. Materials with negative (e) include metals (such as silver, gold, and aluminum) at optical frequencies; materials with negative (m) include resonant ferromagnetic or antiferromagnetic systems.

6) That negative material parameters occur near a resonance has two important consequences. First, negative material parameters will exhibit frequency dispersion: They will vary as a function of frequency. Second, the usable bandwidth of negative materials will be relatively narrow compared with positive materials. These consequences can help answer our initial question as to why materials with (e) and (m) both negative are not readily found. In existing materials, the resonances that give rise to electric polarizations typically occur at very high frequencies -- in the optical for metals, and at least in the terahertz-to-IR region for semiconductors and insulators. On the other hand, resonances in magnetic systems typically occur at much lower frequencies and usually tail off toward the THz and IR region. In short, the fundamental electronic and magnetic processes that give rise to resonant phenomena in materials simply do not occur at the same frequencies, although no physical law would preclude such overlap.(2-5)

References (abridged):

1. V. G. Veselago, Sov. Phys. Usp. 10, 509 (1968)

2. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, IEEE Trans. Microwave Theory Tech. 47, 2075 (1999)

3. R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 77 (2001)

4. A. A. Houck, J. B. Brock, I. L. Chuang, Phys. Rev. Lett. 90, 137401 (2003)

5. C. G. Parazzoli, R. B. Greegor, K. Li, B. E. C. Koltenbah, M. Tanielian, Phys. Rev. Lett. 90, 107401 (2003)

Physics Today http://www.physicstoday.org

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MATERIALS SCIENCE: ON TERAHERTZ MAGNETIC RESPONSE

The following points are made by T.J. Yen et al (Science 2004 303:1494):

1) The range of electromagnetic material response found in nature represents only a small subset of that which is theoretically possible. This limited range can be extended by the use of artificially structured materials, or metamaterials, that exhibit electromagnetic properties not available in naturally occurring materials. For example, artificial electric response has been introduced in metallic wire grids or cell meshes, with the spacing on the order of wavelength (1); a diversity of these meshes are now used in THz optical systems (2).

2) More recently, metamaterials with subwavelength scattering elements have shown negative refraction at microwave frequencies (3), for which both the electric permittivity and the magnetic permeability are simultaneously negative. The negative-index metamaterial relied on an earlier theoretical prediction that an array of nonmagnetic conductive elements could exhibit a strong, resonant response to the magnetic component of an electromagnetic field (4).

3) Conventional materials that exhibit magnetic response are far less common in nature than materials that exhibit electric response, and they are particularly rare at THz and optical frequencies. The reason for this imbalance is fundamental in origin: Magnetic polarization in materials follows indirectly either from the flow of orbital currents or from unpaired electron spins. In magnetic systems, resonant phenomena, analogous to the phonons or collective modes that lead to an enhanced electric response at infrared or higher frequencies, tend to occur at far lower frequencies, resulting in relatively little magnetic material response at THz and higher frequencies.

4) Magnetic response of materials at THz and optical frequencies is particularly important for the implementation of devices such as compact cavities, adaptive lenses, tunable mirrors, isolators, and converters. A few natural magnetic materials that respond above microwave frequencies have been reported. For example, certain ferromagnetic and antiferromagnetic systems exhibit a magnetic response over a frequency range of several hundred gigahertz (5) and even higher. However, the magnetic effects in these materials are typically weak and often exhibit narrow bands, which limits the scope of possible THz devices. The realization of magnetism at THz and higher frequencies will substantially affect THz optics and their applications.

5) In summary: The authors demonstrate that magnetic response at terahertz frequencies can be achieved in a planar structure composed of nonmagnetic conductive resonant elements. The effect is realized over a large bandwidth and can be tuned throughout the terahertz frequency regime by scaling the dimensions of the structure. The authors suggest that artificial magnetic structures, or hybrid structures that combine natural and artificial magnetic materials, can play a key role in terahertz devices.

References (abridged):

1. R. Ulrich, Infrared Phys. 7, 37 (1967)

2. S. T. Chase, R. D. Joseph, Appl. Opt. 22, 1775(1983)

3. R. A. Shelby, D. R. Smith, S. Schultz, Science 292, 79 (2001)

4. J. B. Pendry, A. J. Holden, D. J. Robbins, W. J. Stewart, IEEE Trans. Microw. Theory Tech. 47, 2075(1999)

5. P. Grunberg, F. Metawe, Phys. Rev. Lett. 39, 1561 (1977)

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