Number 750, October 18, 2005
by Phil Schewe and Ben Stein
Particles of Heat
The phonon Hall effect, the acoustic equivalent of the electrical
Hall effect, has been observed by physicists at the Max Planck
Institut für Festkörperforschung (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
degrees Kelvin 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. The same team of MPI-CNRS scientists in 1997 demonstrated a kind of
"photon Hall effect": see
PNU 349
Detecting Alzheimer's Early with Non-Invasive Optical Tools
Building upon the stunning recent discovery that Alzheimer's disease
can be detected early by looking for telltale proteins in the eye,
researchers at this week’s
Frontiers in Optics meeting of the
Optical Society of America presented a pair of optical tests, both
in clinical trials, that can potentially diagnose the disease in its
beginning stages. Such tests may not only improve patients' chances
to start treatment earlier, but they could also speed development of
new Alzheimer's drugs.
Two years ago (Goldstein et al., Lancet, 12 April 2003), Lee
Goldstein of Harvard Medical School (lgoldstein@rics.bwh.harvard.edu)
and his colleagues showed that the
exact same amyloid beta proteins which are a hallmark of Alzheimer's
disease are also found in the lens and its surrounding fluid. In
those portions of the eye, the proteins form amyloid deposits
similar to those in the brain. Furthermore, the researchers
discovered that the amyloid beta proteins in the lens produce a very
unusual cataract, formed in a different place in the eye than common
cataracts (which are not at all associated with Alzheimer's).
Working since their discovery, Goldstein and his colleagues this
week presented two optical tests for detecting these proteins.
Using a technique known as quasi-elastic light scattering, the first
test employs low-power infrared laser light to non-invasively detect
protein particles in the specific part of the lens where these
unusual cataracts form. The second test would be applied to those
who screen positively for the proteins, in order to confirm an
Alzheimer's diagnosis. This test uses a technique Goldstein and
colleagues call "fluorescence ligand scanning" (FLS). The
researchers apply special fluorescing eye drops with image-enhancing
molecules that bind to the amyloid beta molecules. If amyloid beta
molecules are present, the fluorescing molecules will light them up.
The first test is in human and animal trials and the
second test is in animal trials only.
These two diagnostic tests are envisioned to be a two-step process
for screening and then confirming an Alzheimer's diagnosis. These
new optical tools can also potentially speed up the development of
new Alzheimer's drugs, by giving investigators rapid feedback on
whether the drug is doing its job of removing the harmful proteins
from the body. Moreover, the researchers are using the same
technologies to develop new tests for rapidly detecting amyloid
plaques resulting from prion diseases, including mad cow, scrapie in
sheep, and Creutzfeldt-Jacob disease in humans.
Physicists at the University of
Texas at Austin 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
Frontiers in Optics
meeting of the Optical Society of America, consists of a silicon carbide
membrane 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.
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