Revolutionary Tungsten Photonic Crystal Could Provide More Power For Electrical Devices...
Source: Sandia National Laboratories
Revolutionary Tungsten Photonic Crystal Could Provide More Power For
ALBUQUERQUE, N.M. -- You can't get something for nothing, physicists say,
but sometimes a radical innovation can come close.
Researchers at Sandia National Laboratories -- exceeding the predictions of
a 100-year-old law of physics -- have shown that filaments fabricated of
tungsten lattices emit remarkably more energy than solid tungsten filaments
in certain bands of near-infrared wavelengths when heated.
This greater useful output offers the possibility of a superior energy
source to supercharge hybrid electric cars, electric equipment on boats, and
industrial waste-heat-driven electrical generators. The lattices' energy
emissions put more energy into wavelengths used by photovoltaic cells that
change light into electricity to run engines.
Because near-infrared is the wavelength region closest to visible light, the
day may not be distant when tungsten lattice emissions realized at visible
wavelengths provide a foundation for more efficient lighting -- the first
significant change in Edison's light bulb since he invented it.
"This is an important and elegant work," says Cal Tech professor Amnon Yariv
of the research achievement. Yariv is a member of the National Academy of
Engineering and a leading figure in quantum optics research.
The work has been granted two patents with another pending. Two papers
describing the advance have been accepted by the journal Optics Letters.
Another will be published by Applied Physics Letters.
Sub-micron-featured lattices -- which resemble very tiny garden lattices
carefully stacked one atop the other -- can be mass-produced cheaply with
today's computer-chip technologies.
The lattice itself can be visualized as a construction built of a child's
Lincoln Logs. The tungsten "logs" of this experiment have diameters of 0.5
microns separated by distances of 1.5 microns.
The lattices are also known as photonic crystals because of the crystalline
regularity of the spacing of their components. At first such crystals were
of interest because they could bend specific frequencies of light without
loss of energy. This was because the crystal's channels were constructed of
exactly the right dimensions to form a 'home' for particular wavebands as
they travelled. The innovation of the current method is to use the channels
not to bend light but to permit input energy to exit only in the desired
The shadow of Max Planck
The demonstration, led by Sandia physicist Shawn Lin, exceeds in output a
well-known law formulated a century ago by Max Planck, one of the founders
of modern physics. The equation, called Planck's Law of Blackbody Cavity
Radiation, predicts the maximum emissions expected at any wavelength from
The somewhat startling Sandia results exceeded these predictions by four to
10 times at near-infrared frequencies, says Lin.
In terms of electrical output, for the Sandia lattice heated in a vacuum to
1,250 degrees C -- the typical operating temperature of a thermal
photovoltaic generator -- a conversion efficiency of 34 percent was
calculated, three times the performance of an ideal blackbody radiator,
predicted to be 11 percent.
Electrical power density was calculated to be approximately 14 watt/cm
squared, rather than three watt/cm squared expected from an ideal blackbody
No deterioration of the tungsten lattice was observed, although long-term
tests have yet to be run.
Cat vs. supercat
Lin says his group's work does not break Planck's law but only modifies it
by demonstrating the creation of a new class of emitters.
"To compare the amounts of emissions from a solid and a photonic lattice is
like comparing a dog and a cat -- or, a cat and a super cat,"he says.
A photonic lattice apparently subjects energies passing through its links
and cavities to more complex photon-tungsten interactions than Planck dreamt
of when he derived his system that successfully predicted the output
energies of simple heated solids. And a lattice's output is larger than a
solid's only in the frequency bands the lattice's inner dimensions permit
energy to emerge in.
Still, says Kazuaki Sakoda of Japan's Nanomaterials Laboratory at the
National Institute for Materials Science, "One of the most important issues
in contemporary optics is the modification of the nature of the radiation
field and its interaction with matter. [Lin's] recent work clearly
demonstrates that even Planck's law -- the starting point of the era of
quantum mechanics [used to predict these interactions] -- can be modified.
To my knowledge, [Lin's papers] are the first experimental report on this
Sakoda's book, Optical Properties of Photonic Crystals, was published by
Springer Verlag, Berlin, 2001.
Theoretically, there are still unresolved questions as to how the process
works without contradicting other physical laws.
Nevertheless, MIT physics professor John Joannopoulus in response to a
question from a Sandia interviewer had high praise for the work. "It is
definitely not a -- how did you put it -- 'a small step forward,' it's
really a leap forward. It is a very clever completely believable ... I think
it's an exciting experiment, very carefully done, and there's some really
interesting new science here." Joannopoulus is a pioneer in photonic
lattices and wrote the first book on the field.
The scientist at rest
Standing in his equipment-cluttered laboratory, Shawn Lin grins happily
among the vandalized wreckage of a number of ordinary light bulbs from
K-Mart. His team pirates the bulbs' screw-in bases and glass filament
supports for use as cheap, pre-made connectors and supports for the
iridescent slivers of photonic lattice his team substitutes for common
filaments of solid tungsten.
"Look!" Shawn says with obvious anticipation, and flips a switch connected
to where the reconstituted filament sits in a vacuum chamber.
In its little chamber, like a kind of witches' Sabbath for light bulbs, the
bulb, though formerly dead, now glows again, but with a distinctly yellow
light. The lattice filament, powered by only two watts, and with most of its
output keyed to the infrared range at 1.5 to 2 microns, has enough of a tail
into the visible spectrum for the lattice to glow. "We are that far along!"
Lin says with satisfaction.
If these results at 1.5 microns can be extended to the visible spectrum,
ramifications of this work may help form the next generation of lighting
after the currently more mature LED technology.
The increased amount of usable energy available from lattices (also known as
photonic crystals) at specific frequencies is important to engineers dealing
with electricity-driven engines.
A photonic lattice absorbing energies from a power plant generator's excess
heat could release it at higher frequencies readily absorbable by the
photovoltaic cell that powers electricity-driven engines.
While such engines -- best known in the form of electric-powered cars "
exist, their efficiencies have been much lower than hoped because their
receivers cannot absorb incoming energies across the wide spectrum of
infrared radiation generated as unwanted heat but only from limited bands
within the broad range. Here, the lattice could serve as a kind of funnel,
forcing the heat radiation into predetermined frequency bands. When placed
between the generator -- be it solar, dynamo, or fire -- and receiver, the
metallic photonic lattice can be engineered to absorb energies, become
thermally excited, and release them in only a few frequency bands.
While some energy is lost in this process, it makes available energies from
frequencies previously unusable.
Visit the past
A year ago (Nature, May 2, 2002), Lin's team showed that a tungsten lattice
could gather absorbed energies at shorter wavelengths than ordinary tungsten
could. Now, Lin with colleagues Jim Fleming, Jim Moreno (ret.), and Ihab
El-Kady show actual emissions. The emission measurements were performed with
the technical assistance of Jim Bur and Jonathan Rivera. Part of the earlier
simulation of tungsten lattice's optical properties was done at Iowa State
University/Ames National Laboratory, in work led by Professor Kai Ming Ho.
The current use of tiny lattices to emanate energy in designated wave bands
is a conceptual jump from their earliest appearances over a decade ago, when
it was thought their major function would be to bend light without loss for
telecommunications. Such lattices were built from semiconductor materials.
In the case discussed here, semiconductor materials are used to form a
lattice mold into which tungsten is introduced. The semiconductor material
is then etched away, resulting in a thin tungsten photonic crystal sample
about five micrometers thick.
Sandia National Laboratories' World Wide Web home page is located at [Only registered users see links. ]. Sandia news releases, image gallery, and periodicals
can be found at the News and Events button.
Editor's Note: The original news release can be found here.
Do Wah Ditty