Text 757, 121 rader
Skriven 2006-03-23 20:51:17 av Herman Trivilino (1:106/2000.7)
Ärende: PNU 770
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PHYSICS NEWS UPDATE
                                        
The American Institute of Physics Bulletin of Physics News
Number 770   March 23, 2006  by Phillip F. Schewe, Ben Stein

TWO-DIMENSIONAL LIGHT, OR PLASMONS, can be triggered when light
strikes a patterned metallic surface.  Plasmons may well serve as a
proxy for bridging the divide between photonics (high throughput of
data but also relatively large circuit dimensions---1 micron) and
electronics (relatively low throughput but tiny dimensions---tens of
nm).  One might be able to establish a hybrid discipline,
plasmonics, in which light is first converted into plasmons, which
then propagate in a metallic surface but with a wavelength smaller
than the original light; the plasmons could then be processed with
their own two-dimensional optical components (mirrors, waveguides,
lenses, etc.), and later plasmons could be turned back into light or
into electric signals.  To show how this field is shaping up, here
are a few plasmon results from that great international physics
bazaar, the APS March Meeting, which took place last week in
Baltimore.
1. Plasmons in biosensors and cancer therapy: Naomi Halas (Rice
Univ., halas@rice.edu) described how plasmons excited in the surface
of tiny gold-coated rice-grain-shaped particles can act as powerful,
localized sources of light for doing spectroscopy on nearby
bio-molecules. The plasmons's electric fields at the curved ends of
the rice are much more intense than those of the laser light used to
excite the plasmons, and this greatly improves the speed and
accuracy of the spectroscopy.  Tuned a different way, plasmons on
nanoparticles can be used not just for identification but also for
the eradication of cancer cells in rats.
2. Plasmon microscope: Igor Smolyaninov (Univ. Maryland,
smoly@eng.umd.edu) reported that he and his colleagues were able to
image tiny objects lying in a plane with spatial resolution as good
as 60 nm (when mathematical tricks are applied, the resolution
becomes 30 nm) using plasmons that had been excited in that plane by
laser light at a wavelength of 515 nm.  In other words, they achieve
microscopy with a spatial resolution much better than  diffraction
would normally allow; furthermore, this is far-field
microscopy---the light source doesn't have to be located less than a
light-wavelength away from the object.  This work is essentially a
Flatland version of optics.  They use 2D plasmon mirrors and lenses
to help in the imaging and then conduct plasmons away by a
waveguide.
3. Photon-polariton superlensing and giant transmission: Gennady
Shvets (Univ. Texas, gena@physics.utexas.edu) reported on his use of
surface phonons excited by light to achieve super-lens (lensing with
flat-panel materials) microscope resolutions as good as
one-twentieth of a wavelength in the mid-infrared range of light.
He and his colleagues could image subsurface features in a sample,
and they observed what they call "giant transmission," in which
light falls on a surface covered with holes much smaller than the
wavelength of the light.  Even though the total area of the holes is
only 6% of the total surface area, 30% of the light got through,
courtesy of plasmon activity at the holes.
4.  Future plasmon circuits at optical frequencies: Nader Engheta
(Univ. Pennsylvania, engheta@ee.upenn.edu) argued that
nano-particles, some supporting plasmon excitations, could be
configured to act as nm-sized capacitors, resistors, and
inductors---the basic elements of any electrical circuit.  The
circuit in this case would be able to operate not at radio (10^10
Hz) or microwave (10^12 Hz) but at optical (10^15 Hz) frequencies.
This would make possible the miniaturization and  direct processing
of optical signals with nano-antennas, nano-circuit-filters,
nano-waveguides, nano-resonators, and may lead to possible
applications in nano-computing, nano-storage, molecular signaling,
and molecular-optical interfacing.

NANOPORES AND ZEPTOMOLE BIOLOGY.  Some proteins naturally form
nanometer-scale pores that serve as channels for useful biochemical
ions.  Through this ionic communication, nanopores enable many
functions in cells, such as allowing nerve cells to communicate
(they are even responsible for twitching the frog leg in Galvani's
famous discovery in the 1700s).   Nanopores can be destructive too.
When the proteins of bacteria and viruses attach to a cell, their
nanopores can facilitate infection, for example by shooting viral
DNA through them into the cell.  At the APS March Meeting, NIST's
John J. Kasianowicz (john.kasianowicz@nist.gov) showed how single
biological nanopores can be used to detect and characterize
individual molecules of RNA and DNA.  He also demonstrated
constructive uses for anthrax-related nanopores in diagnosing
anthrax infections and testing anti-anthrax drugs.  Anthrax bacteria
secrete a protein called "protective antigen" that attaches to an
organic membrane such as a cell wall.   The protein forms a nanopore
that penetrates the membrane.  When another anthrax protein called
"lethal factor" attaches to the protective antigen nanopore, it
prevents ionic current from flowing through the pore (and out of the
organic membrane). By monitoring animal blood samples for changes in
ion current, Kasianowicz and his colleagues at the National Cancer
Institute and the United States Army Medical Research Institute for
Infectious Diseases electronically detected a complex of two anthrax
proteins in less than an hour, as opposed to the existing methods
which can take up to several days. Also, they demonstrated a method
for screening potential therapeutic agents against anthrax toxins
using the anthrax nanopore (see
http://www.nist.gov/public_affairs/techbeat/tb2005_0826.htm#anthrax
for a picture and more information).
A Brown University group led by Sean Ling (Xinsheng_Ling@brown.edu)
was among those reporting progress in developing a nanopore-based
method for sequencing DNA faster and more cheaply than traditional
biochemical techniques.  In one scenario the change in ion current
as DNA moves through the nanopore could yield the sequence of bases
(letters) in the DNA.  However, the letters in DNA are so close to
each other (about 4 angstroms) and the DNA moves so quickly through
the nanopore that researchers have had to come up with creative
solutions for reading the individual letters.  For example, the
Brown group attaches complementary blocks of DNA, about 6 letters
long, to the DNA sequence of interest, so that the researchers would
read blocks of multiple letters at a time, while slowing down the
passage of the DNA by attaching a magnetic bead to it.
Other researchers are finding value in developing nanopores for
fundamental biology studies.  Discussing his group's latest work
with artificial, silicon-based nanopores, Cees Dekker of the Delft
University of Technology (dekker@mb.tn.tudelft.nl) showed how lasers
and other manipulations with the artificial pores are enabling new
single-molecule (zeptomolar) biophysics studies on the properties of
DNA, RNA, and proteins by studying how they pass through the pores
(see www.aip.org/png for an artist's rendering of DNA traversing
through a nanopore)

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 * Origin: Big Bang (1:106/2000.7)