ZPEnergy.com - THE PHONON HALL EFFECT; SUPER LENSING IN THE MID INFRARED

PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 750 October 19, 2005 by Phillip F. Schewe, Ben Stein

THE PHONON HALL EFFECT, the acoustic equivalent of the electrical Hall effect, has been observed by physicists at the Max Planck Institut fur Festkorperforschung (MPI) and the Centre National de la Recherche Scientifique (CNRS) in France. In the electrical Hall effect, when an electrical current (consisting of free electrons moving along a material sample) being driven by an electric field is subjected to an external magnetic field, the charge carriers will feel a force perpendicular to both the original current and the magnetic force, causing the electrical current to be deflected somewhat to the side.

Thermal transport is a bit more complicated than electrical transport. A "current" of heat can consist of free electrons carrying thermal energy or it can consist of phonons, which are vibrations rippling through the lattice of atoms of the sample.

Previously, some scientists believed that in the absence of free electrons, a magnetically induced deflection of heat could not be possible. The MPI-CNRS researchers felt, however, that a magnetic deflection of phonons was possible, and have now demonstrated it experimentally in insulating samples of Terbium Gallium Garnet (a material often used for its magneto optical properties) where no free charges are present. The sample was held at a temperature of 5 K and was warmed at one side, creating the thermal equivalent of an applied voltage. Application of a magnetic field of a few Tesla led to an extremely small (smaller than one thousandth of a degree) yet detectable temperature difference.

(Strohm et al., Physical Review Letters, 7 October 2005; text at www.aip.org/physnews/select) The same team of MPI-CNRS scientists earlier demonstrated a kind of "photon Hall effect"
(http://www.aip.org/pnu/1997/split/pnu349-2.htm).

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SUPER LENSING IN THE MID INFRARED. Physicists at the University of Texas have made a "super lens," a plane-shaped lens that can image a point source of light down to a focal spot only one-eighth of a wavelength wide; this is the first time such super lensing has been accomplished in a functional device in the mid-infrared range of the electromagnetic spectrum.

Historically lensing required a lens-shaped (that is, lozenge-shaped) optical medium for bringing the diverging rays coming from a point source into focus on the far side of the lens.

But in recent years, researchers have found that in "negative permittivity" materials (in which a material's response to an applied electric field is opposite that of most normal materials), light rays can be refracted in such a way as to focus planar waves into nearly a point, albeit over a very truncated region, usually only a tenth or so of the wavelength of the light.

Such near-field optics are not suitable for such applications as reading glasses or telescopes, but have become an important technique for certain kinds of nanoscale imaging of large biological molecules than can be damaged by UV light. The micron-sized Texas lens, reported at the OSA meeting
(http://www.osa.org/meetings/annual), consists of a silicon carbide membrane in between layers of silicon oxide. It focuses
11-micron-wavelength light, but the researchers hope to push on into the near-infrared range soon. Furthermore, the lensing effect seems to be highly sensitive to the imaging wavelength and to the lens thickness.

Gennady Shvets (gena@physics.utexas.edu) says that additional possible applications of the lens include direct laser nanolithography and making tiny antennas for mid-IR-wavelength free-space telecommunications. (Paper fMG2 at meeting; Lab website: www.ph.utexas.edu/~shvetsgr/)


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