PHYSICSWATCH Compiled by Steve Reucroft and John Swain, Northeastern University

Metallic 'picket fence' cuts the speed of light Materials in which light slows down are usually thought of as being fairly uniform, like water or glass. Now, however, Jung-Tsung Shen, Peter Catrysse and Shanhui Fan of Stanford University, California, have suggested a novel way to produce refraction, involving a periodic arrangement of metal films. Moreover, the resulting refractive index, which is positive, does not depend on frequency, and can be arbitrarily large.

The idea is to pass light through a miniature picket-fence-like structure made of metal slats that are thicker than the spaces between them, and with gaps smaller than the wavelength of light. While there is no electric field possible inside the metal, waves with a magnetic field parallel to the slats can get through, even though the wavelength is much larger than the space between the slats. The structure then acts like a refractive material -in effect, a dielectric slab with a refractive index, n, equal to the width of the slats, d, divided by the distance between them, a.

This means that the refractive index can be tuned by changing the geometry of the structure, independent of frequency. In particular, a positive index of refraction can be made arbitrarily large, with potential

Calculated variations in the magnetic field in fundamental waveguide modes show the equivalence of a slotted metal film (top) and the corresponding dielectric slab for a refractive index of 4. (Red indicates positive amplitude, while blue is negative.)

applications in the miniaturization of optical or electromagnetic devices, and in imaging.

Further reading J-T Shen, Peter Catrysse and Shanhui Fan 2005 Phys. Rev. Lett. 94 197401.

Special relativity becomes more general Special relativity arises, in a basic sense, from the introduction of an invariant velocity - the speed of light - and, as every physicist knows, this requires a radical revision of the nature of space and time. Recent works, however, have suggested the existence of other invariant scales, leading to so-called doubly and triply special relativities. Now Dharam V Ahluwalia-Khalilova of the University of Zacatecas, Mexico, has pulled many of these suggestions together, while arguing that nonlinear deformations are not needed. He finds that, to have a stable algebraic structure, one is led to two invariant length scales. One is small and perhaps related to the Planck length,

while the other is large and presumably related to the size of the universe.

The word "stable" here indicates that the structure can withstand small changes, as special relativity can withstand small changes in the speed of light without becoming a fundamentally different theory. In a sense, this is about as general a theory of special relativity as there is, and can test special relativity with just two free parameters. Now the challenge is to think of some good experimental tests.

Further reading D V Ahluwalia-Khalilova 2005 Class. Quant. Grav. 22 1433.

Handheld fusion device generates a future in neutrons A novel way to generate neutrons using fusion in a handheld device could find many applications where modest numbers of neutrons are useful. Brian Naranjo and colleagues have found that the key is a pyroelectric crystal attached to a tungsten probe and immersed in low-pressure deuterium gas.

On heating, the crystal generates remarkable charge separations, which translate to an electric field of an amazing 25 V/nm at the tungsten probe. This field ionizes deuterium and then repels the ions into a target of erbium deuteride where deuterium-deuterium fusion produces a neutron and a helium-3 nucleus.

This is unlikely to be used to generate energy via fusion, but it could lead to convenient sources of thousands to millions of neutrons.

Further reading B Naranjo etal. 2005 Nature 434 1115.

How cerium-based metallic glasses show their soft side Metals with low melting points, or which at least become plastic at low temperatures, are always of technological interest. Now Wei Hua Wang and colleagues at the Institute of Physics, Beijing, and Cambridge University have found a class of cerium-based metallic glasses with a glass-transition temperature so low that in boiling water they become soft like putty.

The glass transition is gentle with malleability starting at 68 °C, and the materials show remarkable resistance to crystallization. This makes them attractive both for practical applications and for studies of glass transition.

Further reading B Zhang ef. al. 2005 Phys. Rev. Lett. 94 205502.

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