Number 835, August 3, 2007
by Phil Schewe and Ben Stein
Opaque Lens
Physicists at the University of Twente in the Netherlands are able to focus a beam of light by sending it through an opaque medium. Normally an opaque substance, such as milk or paint, will only serve to scatter light waves. But by carefully sculpting the incoming laser beam-processing tiny portions of the forward-moving wavefront---the Dutch researchers were able to focus the beam to an intensity 1000 times brighter than for the normal diffuse transmission at that same point previously.
They do this by first sending the light through the medium and, with a CCD camera, recording the attenuation of the beam at various points behind the sample. From this one can calculate scattering coefficients corresponding to the degree of statistical scattering across the face of the sample. An optical device, called a phase modulator, is used to anticipate and correct for the scattering yet to occur on a point-by-point basis.
To see a movie of the before- and after-modulator light scattering, go to cops.tnw.utwente.nl/research/animation_focus/speckle.html.
The titanium-oxide sample normally admits light at the 10% level (through the entire sample), but with the modulator in place, the transmission goes way up.
One of the Twente researchers, Ivo Vellekoop (i.m.vellekoop@tnw.utwente.nl, 31-53489-5390), says that the diffusion of light in a dispersive medium is, after this, not an inevitability, and that spectroscopy and microscopy (even of single cells embedded in human tissue) should be greatly improvable. (Vellekoop and Mosk, Optics Letters, 15 August 2007)
A Superconducting Ferromagnet at Normal Pressure
A superconducting ferromagnet at normal pressure has been devised from the elements uranium, cobalt, and germanium by physicists from the University of Amsterdam and the University of Karlsruhe. Normally magnetism is anathema to the delicate pairing of electrons at the heart of the superconducting phenomenon.
This would be especially true for ferromagnetism, that sturdiest of magnetic states (in which atoms aligned by an external magnet retain their coordination even when the external field is removed). A few years ago several materials were found that supported superconductivity and ferromagnetism, but only under conditions of high pressure or extremely cold temperatures (see Physics Today, Sept 2001).
What happens is that instead of the normal pairing of two electrons with opposite spins (one spin up and one spin down-a “spin-singlet” state) in these ferromagnetic-tolerant materials the pairings involve electrons whose spins are in the same direction (a spin-triplet state). The material remains superconducting and ferromagnetic both under ordinary pressures, and at a temperature, 1 K, about four times higher than for any other ambient-pressure material.
According to one of the researchers, Anne de Visser (devisser@science.uva.nl), the presence of the magnetic fluctuations needed to preserve the ferromagnetic state might complicate, in an interesting way, the BCS mechanism normally at work in low-temperature superconductors. (Huy et al., Physical Review Letters, upcoming article; lab website, http://www.science.uva.nl/research/cmp/devisser/)
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