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Looking more generally, it is clear that boundaries and layers are often a necessity in biological systems. For example, layers of different cell types interact and communicate to influence cell differentiation and tissue formation6. Gradients of growth factors prove critical for innumerable actions during inflammation of tissues and wound healing7. In regenerative medicine, researchers often attempt to mimic the native structure of a tissue using biomaterials to help organize cells appropriately. For example, a blood vessel has three different layers, with each layer containing different cells and extracellular matrix. As a result of the flexibility of

Ladet and colleagues’ technique, a tube (Fig. 1a) could be created where the different cell types are introduced in between the membrane layers.

From a more basic perspective, biologists are often interested in learning how cells ‘talk’ to each other in a controlled in vitro environment. The ability to co-culture different cell types together in a three-dimensional environment, in separate adjacent layers, will allow scientists to evaluate more realistic cell–cell interactions8.

The general applicability of the system is demonstrated with the creation of layered hydrogels from both chitosan and alginate, which are cationic and anionic

polyelectrolytes, respectively. Most importantly, these model biopolymers are widely accessible, making this multilayered hydrogel technology within practical reach of the general scientific community.

references1. Peppas, N. Hydrogels in Medicine and Pharmacy (CRC,

Boca Raton, Florida, 1987).2. Ladet, S., David, L. & Domard, A. Nature

452, 76–79 (2008).3. van der Linden, H., Olthuis, W. & Bergveld, P. Lab Chip

4, 619–624 (2004).4. Berger, J. et al. Eur. J. Pharm. Biopharm. 57, 35–52 (2004).5. Chen, R. R. et al. FASEB J. 21, 3896–3903 (2007).6. Dormann, D. & Weijer, C. J. Curr. Opin. Genet. Dev.

13, 358–364 (2003).7. Baker, R. E. & Maini, P. K. Math. Biosci. 209, 30–50 (2007).8. Sharma, B. et al. Tissue Eng. 13, 405–414 (2007).

PHoToniCs

Metamaterials to beat the static

evgenii narimanovis in the School of Electrical and Computer Engineering, Purdue University, West Lafayette, Indiana 47907, USA.

e-mail: evgenii@purdue.edu

The development of electromagnetic metamaterials, artificial structures patterned on a scale much smaller

than the wavelength of light they interact with, has uncovered a whole new realm of physical phenomena. The unparalleled flexibility in the design of the ‘meta-atoms’ — the unit cells of the metamaterial structure — has led to the realization of properties not found in nature, such as negative refractive index1 and electromagnetic cloaking2. Imaging devices based on metamaterials, such as the superlens3, can break the diffraction limit of optical imaging. On page 295 of this issue, Fridrik Magnus and colleagues demonstrate a new approach to metamaterials and their applications by fabricating a superconducting metamaterial4.

To achieve such exotic properties in controlling electromagnetic fields, metamaterials generally incorporate metal structures. Such metamaterials, however, show high absorption losses due to interaction of light with the free electrons in the metal. Unfortunately, this

typical response of metals is exacerbated as metamaterials rely on the resonant response of the meta-atoms to light — with resulting dramatic enhancement of the loss. Even in the best currently available samples of optical negative index metamaterials5, the imaginary part of the refractive index is comparable to its real part. This means that the absorption of the material leads to a significant drop in signal intensity within a length

on the order of the wavelength. As it is precisely those resonances that are generally responsible for the unusual properties of metamaterials, the issue of losses is considered the Achilles heel of metamaterials that drastically limits their practical applications.

Although these losses may seem to be a fundamental obstacle, in metamaterials that operate at low frequencies this problem can actually be resolved — if the

The development of superconducting metamaterials opens the way to a new level of control over electromagnetic fields.

Figure 1 superconducting metamaterials. a, a schematic of the metamaterial formed by an array of superconducting cubes. reprinted with permission from ref. 6. b, a section of the structure subject to a uniform magnetic field (red).

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metamaterials in question are designed around superconducting elements6. Although the a.c. operation of such superconducting metamaterials will not be entirely loss-less, for low frequencies the corresponding absorption in such systems will be significantly smaller than in their resistive metal counterparts.

Following on the original theoretical work by Wood and Pendry6, Magnus and colleagues have now demonstrated the first such superconducting metamaterial4. As opposed to the inherent electromagnetic resonant response harvested by ‘conventional’ metamaterials5, this novel design is essentially non-resonant. Instead, the function of the metamaterial relies on the expulsion of the magnetic field from the superconducting elements. If the metamaterial is formed by an array of superconducting cubes (Fig. 1a), the magnetic field, applied in the direction of one of the cube edges, is confined to the gaps between the faces parallel to the field direction (Fig. 1b). As has been shown previously, this confinement modifies the magnetic permeability of the material6.

This modified magnetic response depends on the size of the cubes as well as the separation between them, and can therefore be controlled by the geometry of the unit cell of the metamaterial.

Using different and more complex structures than the present case thus offers another control parameter for extending the range of the magnetic response of the system, tailored for specific applications.

In their experimental realization of superconducting metamaterial, Magnus et al. used stacked layers of an array of lead plates in sizes ranging from 133 to 163 µm. Depending on the size of the superconducting elements and the gaps between them, the observed magnetic permeability varied between zero and about 0.6, in perfect agreement with the original theoretical prediction.

The successful demonstration of this concept may now enable local control of the magnetic response by the variation of the unit-cell geometry across the device structure. Such inhomogeneous metamaterials may lead to a much more sophisticated way of control

over electromagnetic fields, termed ‘transformation optics’.

In particular, the superconducting metamaterials pioneered by Magnus and collaborators can be used for ‘cloaking’ a region of space from static magnetic fields while — unlike the common Meissner effect of magnetic field expulsion from bulk superconductors — leaving the outside fields unperturbed. Furthermore, superconducting metamaterials also offer a new system for exploring the interplay of ‘classical’ electromagnetism with quantum effects such as Josephson tunnelling. Although it is too early to tell which devices and applications will take the most advantage of superconducting metamaterials, their development opens up many exciting possibilities.

references1. Veselago, V. G. Sov. Phys. USPEKHI 10, 509–514 (1968).2. Pendry, J. B., Schurig, D. & Smith, D. R. Science

312, 1780–1782 (2006).3. Pendry, J. B. Phys. Rev. Lett. 85, 3966–3969 (2000).4. Magnus, F. et al. Nature Mater. 7, 295–297 (2008).5. Shalaev, V. M. Nature Photon. 1, 41–48 (2007).6. Wood, B. & Pendry, J. B. J. Phys. Condens. Matter

19, 076208 (2007).

Some new fields of research announce themselves with a fanfare — high-temperature superconductivity was one. Others start with a whisper, heard by insiders but inaudible to the rest of us. Plasmonics is of this variety. Had this field begun with what is now its most famous

trick — rendering objects invisible with shields that bend electromagnetic radiation in strange ways — it would have made instant headlines. But one of the foundational papers seemed at the time a mere curiosity, not obviously fitting into any familiar agenda.

Thomas Ebbesen and his co-workers at the NEC Research Institute in Princeton reported in 1998 that light can be squeezed through metallic films perforated by arrays of cylindrical pores, even though the pores were just a tenth as wide as the light’s wavelength (Nature 391, 667–669; 1998). That defied the conventional wisdom that light transmission is limited by diffraction effects. Ebbesen discovered this almost a decade earlier, but only later deduced

what was happening: the light excites waves in mobile surface electrons of the metal (plasmons), with the same frequency but shorter wavelength. These can pass through the holes and lead to re-radiation of light on the far side.

Ebbesen and colleagues hinted at “applications in novel photonic devices”, but only more recently has it become clear what that could mean: for example, coupling light signals to nanoscale metal waveguides on a chip without suffering from diffraction-limit restrictions on size. Plasmonic nanoshells have been proposed as light-absorbing heaters to burn up cancerous tissue. And metamaterials, the stuff of the invisibility cloak, also depend on the excitation of surface plasmons in their metallic components.

One problem with plasmonic devices is that some of the plasmon energy is typically radiated away because of scattering at the boundaries of materials with different refractive index. Elser and Podolskiy have now shown how metamaterials might do away with this source of power loss, by cladding interfaces with structures of suitably graded optical properties — not unlike the way graded-index cladding

on optical fibres stops light escaping (Phys. Rev. Lett. 100, 066402; 2008). They say that electric fields can dynamically tune the local refractive index of these structures, so that a block of metamaterial might be reversibly imprinted with all kinds of optical-response patterns: one moment a lens, the next a mirror or waveguide.

Meanwhile, Le Perchec et al. have shown that surface plasmons could explain why some metal films (notably silver) absorb light strongly: relatively shallow surface grooves can ‘capture’ the light by acting as plasmonic resonators and waveguides (Phys. Rev. Lett. 100, 066408; 2008). The effect might also explain why such metal films produce surface-enhanced Raman scattering, a useful spectroscopic tool. And Zhang et al. have found that surface voltages excited on the inner face of invisibility shields will reflect electromagnetic waves radiated from an active device inside, eliminating leakages that would reveal its presence (Phys. Rev. Lett. 100, 063904; 2008). As far as plasmons go, we are clearly still just scratching the surface.

Philip Ball

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