HOUSTON,
April 27, 2006 – Frequently, the unexpected results
in science are the most exciting. That's the case
with the latest findings from the lab of Rice University's
electrical engineer Daniel Mittleman, who was trying
to find new ways to use terahertz energy, or T-rays,
for chemical sensing when he noticed a strange tendency
of the signals to travel slower if they were sent
down smaller metal wires.
Mittleman and graduate student Kanglin Wang reported
their findings in the April 21 issue of Physical
Review Letters. Their explanation for the odd phenomenon
arises from the unique way that T-rays interact with
the sea of electrons flowing across the surface of
the metal wire.
"A similar variation in wave velocity is well-documented
for higher frequency radiation in the visible portion
of the spectrum, but this was a real puzzle because
no one had predicted it for such low frequencies," said
Mittleman, associate professor of electrical and
computer engineering.
Mittleman and Wang discovered the phenomenon during
follow-up experiments to last year's groundbreaking
development of the first T-ray wire waveguides. Their
discovery that T-rays propogate down bare metal wires
has allowed them to make T-ray endoscopes that can
carry T-rays around corners and into tight places – like
pipes and metal containers – where it hasn't been
feasible to place a T-ray generator. Mittleman hopes
to use the technique to design a new class of chemical
sensors that port security officers can use to quickly
determine whether explosives are hidden inside shipping
containers.
That kind of sensing is possible because of the
unique properties of T-rays, which fall between microwaves
and infrared light in the least-explored region of
the electromagnetic spectrum. Metals and other electrical
conductors are opaque to T-rays, but like X-rays,
T-rays can penetrate plastic, vinyl, paper, dry timber
and glass and unlike X-rays, T-rays pose no health
hazards.
The reason bare metal wires can be used as T-ray
waveguides has to do with the way that light from
the terahertz frequency interacts with the sea of
electrons flowing over the surface of the wire. When
a wave of light strikes the wire, it creates a corresponding
wave, called a plasmon, in the electrons flowing
over the wire's surface.
A new field of optics dedicated to the study of
plasmons has sprung up within the past decade, and
Rice boasts at least a half-dozen leading plasmonics
labs, most of which are dedicated to the design,
testing and use of metallic nanoparticles that are
tailored to interact in particular ways with specific
wavelengths of light.
Mittleman said plasmonics is the key to understanding
the movement of T-rays down metal wires. When T-rays
strike the metal wire, they create plasmons, and
it is via these electron waves that the T-ray energy
propogates down the wire. As the diameter of the
wire gets smaller, the curvature becomes more pronounced,
and this changes the plasmonic properties of the
wire. It is this curvature, coupled with properties
of the metal itself, that causes the T-rays to move
slower.
"This is but one example of the interesting new
physics that coming out of T-ray labs across the
country, and with more researchers taking an interest
in T-rays I think we're well on our way to answering
some of the fundamental questions that must be addressed
for the field to progress," Mittleman said.
The research is funded by Advanced Micro Devices
Inc., the Welch Foundation and the National Science
Foundation,
Contact: Jade Boyd
jadeboyd@rice.edu
713-348-6778
Rice University
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