Shock waves tune light
Technology Research News
Researchers from the Massachusetts Institute
of Technology have used a computer simulation to show that sending shock
waves through photonic crystals could lead to faster and cheaper telecommunications
devices, more efficient solar cells, and advances in quantum computing.
Photonic crystals are lattice structures or solids punctured with
holes. These periodic patterns bend light so that only certain wavelengths
can pass through.
The researchers showed that when a shock wave moves through a
photonic crystal, it temporarily changes the spacings in a way that induces
two surprising changes: a large Doppler shift, and a bandwidth narrowing.
These effects "occur when light interacts with a photonic crystal while
the geometry is actually changing," said Evan Reed, a researcher at MIT.
A physical shock wave can be produced by hurling a projectile
or shooting a high-intensity laser at a crystal. A shock-wave-like effect
can also be produced using sound waves or electricity.
The Doppler shift changes the frequency of lightwaves in a way
similar to the familiar pitch change that happens in sound waves when
a train goes by. The shock wave effect changes the size, or frequency,
of lightwaves fairly drastically. The shift is 10,000 times greater than
usual -- enough that the color of the lightwave changes visibly, according
The effect can be used to efficiently convert light to frequencies
that are useful for communications devices. High-speed communications
signals carried over fiber-optic lines are made up of specific frequencies
of light. Some frequencies travel more efficiently than others, and multiple
signals made up of slightly different frequencies can be sent over the
same line at the same time. The shock wave light-shift effect could be
used to convert lightwaves generated by inexpensive light sources to frequencies
carried more efficiently by optical fibers.
The second effect could lead to more efficient conversion of light
to electricity. The shock wave efficiently narrows the bandwidth of the
spectrum of light, an unusual effect that is potentially useful for harvesting
solar energy, said Reed. "There are many physical systems that increase
the bandwidth of light, but to our knowledge no existing classical systems
are capable of narrowing the bandwidth of" any lightwave, he said.
The simulations showed that the bandwidth of a lightwave can be
narrowed by squeezing it between a shock wave and a reflective surface.
The method could be used to narrow the spectrum of solar lightwaves
entering photovoltaic materials, allowing the materials to convert more
of the energy contained in the full spectrum of sunlight to electricity,
according to Reed.
The frequency-shifting and bandwidth-narrowing effects are largely
independent of the speed, shape and thickness of the shock wave, according
to the researchers.
The researchers' simulations showed that the effects can be produced
without literally using shock waves, which would be impractical. Anything
that changes the photonic crystal's structure as a lightwave propagates
through it will work, according to the researchers. "Methods involving
nonlinear optical materials, acousto-optical materials or [microelectromechanical
systems] devices are likely to be more useful than shock waves due to
their non-destructive and repeatable nature," said Reed.
The effects could be used in some practical applications within
a year, said Reed. Others will require 2-5 years.
The effects could also boost single-photon quantum information
processing efforts, like quantum cryptography, according to Reed. Quantum
computers use properties of particles like photons and atoms to represent
information. Because photons are not absorbed and reemitted by the shock-wave-altered
crystal, in theory, these quantum properties and thus information they
represent can be preserved.
Quantum computers are theoretically much faster at certain tasks
like searching large databases and cracking codes than any possible classical
Reed's research colleagues were Marin Soljacic and John Joannopoulos.
The work is scheduled to appear in an upcoming issue of Physical Review
Letters. The research was funded by the National Science Foundation (NSF).
Timeline: > 1 year, 2-5 years
TRN Categories: Optical Computing, Optoelectronics and Photonics;
Materials Science and Engineering
Story Type: News
Related Elements: Technical paper, "The Color of Shock Waves
in Photonic Crystals," slated to appear in Physical Review Letters.
June 4/11, 2003
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