RESEARCH NEWS

JAN-FEB 2008 | VOLUME 11 | NUMBER 1-2 13

Research into metamaterials has given rise to such

fantastic notions as better-than-diffraction-limit lenses

and invisibility cloaks. But when married with the

principles of slow light propagation, metamaterials

– materials with regular structure on the same length

scales as radiation propagating through them – can

also stop a rainbow in its tracks.

A new theoretical study by researchers at the

Universities of Surrey and Salford in the UK proposes

that a metamaterial structure can stop various

frequencies of light altogether at different points: a

trapped rainbow [Tsakmakidis et al., Nature (2007)

450, 397].

The trick to the process is to exploit a phenomenon

called the Goos-Hänchen shift, in which polarized

light encountering an interface between two media

– in this case, the interface between normal and

negative refractive index materials – can travel

along the interface. A suitably designed interface can

even force light to travel backwards. A study of the

propagation of various modes and their dispersion

relations shows that some modes have a group

velocity dependent on the width of the metamaterial

waveguide.

To this end, Ortwin Hess and coworkers propose a

wedge-shaped negative refractive index metamaterial

layer buttressed by two layers with normal refractive

index. “Before coming to a halt, the light effectively

takes three steps forward and two steps backward

while becoming slower and slower,” he says. “At the

critical width of the tapered layer the light is trapped.”

Such trapped light represents a great advance toward

optical computing, which has until now been held back

by the difficult nature of taming photons.

“The impressive advances in the nanotechnologies

have and are providing a very fertile ground for the

actual realization of our proposed idea,” says Hess.

“The main challenge is to overcome inherent losses in

metamaterials.”

D. Jason Palmer

A collaboration between the University

of Cambridge, UK, and the US Army

has made a huge step forward in the

search for carbon nanotube (CNT)-

based fibers and cables that maintain

the outstanding physical properties

of single nanotubes [Koziol et al.,

Sciencexpress (2007) doi: 10.1126/

science.1147635].

The simple, one-step process

accomplishes the two primary goals

of turning nanotubes into strong

macroscopic fibers. One is reducing

defects in the individual nanotubes,

while maintaining their structural

integrity at the nanoscale, and the

other is keeping the CNTs aligned

along the same direction, maximizing

contact area to reduce slippage.

The process starts with a standard

hydrocarbon feedstock such as hexane,

resulting in double-walled CNTs of

7–8 nm diameter and lengths up to

1 mm. The fibers are spun

continuously from the gas phase into

an aerogel, which is pulled by means

of a metal rod out of the open end of

the furnace, stretching the nanotubes

into a thread.

The best samples show tensile

strengths of some 10 GPa, the

highest ever demonstrated for CNT-

based macroscopic cables. The fibers

maintain their integrity up to 300°C,

and the low defect density means

that the fibers’ electrical and thermal

conductivities remain high.

The result has created a stir, in

particular for military applications.

“There is huge interest,” says Alan

Windle of the University of Cambridge.

A day’s yield of several kilometers

of one-filament fiber weighs 0.1 g,

with the researchers conceding that

kilogram quantities will be required at

the industrial scale. But the approach

is promising. “There is so much to do

and so many questions the material

should fuel academic research interests

for decades ahead,” he says.

D. Jason Palmer

Cable carbonCARBON

New building blocks for nanoparticle superlattices

Scientists from Lawrence Berkeley National Laboratory have

proposed a way to simplify the fabrication of nanoparticle

(NP) superlattices containing three or more types of particle,

extending the range of possible materials that can be formed

[Shevchenko et al., Adv. Mater. (2007) 19, 4183].

Different kinds of NP – metal, semiconductor, and magnetic

– can be mixed to obtain superlattice structures with tailor-

made properties. In inorganic chemistry, solids composed of

three or more types of atoms can give rise to completely new

phenomenon, such as high-temperature superconductivity. It

is hoped that ternary NP-derived solids will yield a similarly

rich range of behaviors. So far, however, mixing just two types

of NP has proved challenging, owing to the complexity of

interactions between the particles.

Mixing three types of NP in a repeatable fashion would be

nearly impossible with current knowledge, because the size,

shape, and chemical interaction of all three different types

of particle must be controlled simultaneously. Sidestepping

this problem, Elena V. Shevchenko and colleagues suggest

combining two NPs to make a building block, which can then

be treated as a single particle when assembling the ternary

superlattice. “The advantage of this approach is that we

only have to control the surface chemistry of two types of

nanoparticles,” explains Shevchenko.

The scientists used a composite NP consisting of a hollow

shell of Fe oxide with a central metallic Fe core. It was mixed

with Au NPs to create a number of different superlattice

structures, depending on the particle ratios, solution

chemistry, and temperature. Not all structures are close

packed, indicating that it is not simply the lowest energy

structure that is formed, but that various interactions

determine the structure.

“In the case of NPs, coulombic interactions compete with

dipolar and van der Waals forces. Roughly equal contributions

of these interactions leads to a diversity of structures,” says

Shevchenko. Reducing a three-component system to a binary

one makes more detailed study of these interactions possible.

Pauline Rigby

NANOTECHNOLOGY

In the negative index core layer, a p-polarized wave

experiences negative Goos-Hänchen shifts at the

interfaces, becoming slower and slower. (Courtesy K. L.

Tsakmakidis and O. Hess.)

Trapping the light fantasticOPTICAL MATERIALS

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