Movie captures trapped light
Technology Research News
have been able to capture and slow light for several years. Slow light,
once better understood, could be used to improve devices like sensors and
optical communications equipment.
Researchers from the University of Twente in the Netherlands, the
University of St. Andrews in Scotland, Ghent University in Belgium, and
the FOM-Institute for Atomic and Molecular Physics in the Netherlands have
moved the field forward with a way to directly observe the phenomenon.
The researchers used a photonic crystal waveguide to slow light
by several orders of magnitude. The waveguide is constructed of semiconductor
material punched with a pattern of holes. The pattern causes lightwaves
to interfere with each other enough to slow the pulse to just under one
micron per three trillionths of a second, or picoseconds.
The pulse traveled about one thousand times slower than the speed
of light in a vacuum, which is 299,792,458 meters per second.
The pattern in the researchers' photonic crystal channels light
from one end of the crystal to the other and briefly traps a portion of
a light pulse in an area near the entrance of waveguide. "Intriguingly,
it is open on the input and output side and would therefore seem to allow
a very easy escape" for the lightwave, said Henkjan Gersen, one of the University
of Twente researchers who is now an assistant research professor at the
University of Aarhus in Denmark. "Despite its open nature, the structure
traps light due to extreme interference effects."
Light-trapping ordinarily requires one or more cavities, or tiny
echo chambers that capture light by causing it to bounce back and forth.
The researchers' waveguide is a 220-nanometer-thick wafer of silicon
with 260-nanometer-diameter holes arranged in a hexagonal pattern. A central,
solid strip that is as wide as three rows of holes forms the waveguide's
The researchers launched infrared pulses lasting 120 millionths
of a billionth of a second, and used a time-resolved near-field microscope
to observe each pulse throughout its trip through the photonic crystal.
Near-field lenses are positioned closer to a sample than the wavelength
of the light used and can be used to observe light that remains on the surface
of a sample. The microscope has a resolution of 240 nanometers -- small
enough to observe details of the shape and position of a light pulse.
The method produces images fast enough to yield a handful of frames
as the light propagates through the waveguide to capture the motion in a
movie. "We demonstrated... that it is possible to directly peek inside a
real photonic crystal waveguide... and observe effects that would have otherwise
remained hidden," said Gersen. "This [will] help scientists to understand
the many fascinating properties of slow light propagation."
Knowledge about slow light effects promises to improve sensors.
Slowing light allows it to interact longer with its surroundings, giving
it more time to react to concentrations of molecules, said Gersen. "Its
ability to sense low concentrations of molecules increases," he said.
The effects could also be used in optical chips that enable faster
telecommunications. "Timing control of optical signals is crucial for increasing
data rates and telecommunication," said Gersen. Slow-light photonic crystal
structures could lead to a new generation of optical chips, he said.
The researchers' waveguide captured about five percent of the energy
contained in the light pulse. This must be increased to make the light traps
practical, said Gersen. Such devices could be ready for practical use in
two to five years, he said.
Gersen's research colleagues were T. J. Karle, R. J. P. Engelen,
W. Bogaerts, J. P. Korterik, N. F. van Hulst, T. F. Krauss, and L. Kuipers.
The work appeared in the February 25, 2005 issue of Physical Review Letters.
The research was funded by the University of Twente, the Netherlands Organization
for Scientific Research, the European Union and Agilent Technologies.
Timeline: 2-5 years
Funding: Corporate, Government, Private
TRN Categories: Optical Computing, Optoelectronics and Photonics
Story Type: News
Related Elements: Technical paper, "Real-Space Observation
of Ultraslow Light in Photonic Crystal Waveguides," Physical Review Letters,
Feb. 25, 2005
June 1/8, 2005
Camera sees behind objects
Movie captures trapped
Speedy photon detector
How It Works:
Nano LEDs made easier
Research News Roundup
Research Watch blog
View from the High Ground Q&A
How It Works
News | Blog
Buy an ad link