Researchers use semiconductors to set speed limit on light

Berkeley - They have shown for the first time that the group velocity of
light - the speed at which a laser pulse travels along a light wave -
can be slowed to about 6 miles per second in semiconductors. While that
speed is not exactly the pace of a turtle, it is 31,000 times slower
than the 186,000 miles (or 300 million meters) per second that light
normally clocks while traveling through a vacuum.

"It's about twice as fast as an orbiting space shuttle," said
Connie J. Chang-Hasnain, UC Berkeley professor of electrical
engineering and computer science and principal investigator of the
project. "This achievement marks a major milestone on the road to ever
faster optical networks and higher performance communications."

"It's about twice as fast as an orbiting space shuttle"The researchers envision a future of 3-D graphics transmission,
high-resolution video conferencing as good as face-to-face encounters
and quantum memory chips that could boost the power of supercomputers,
including those used for complex climate modeling.

Chang-Hasnain and other researchers at UC Berkeley's
Department of Electrical Engineering and Computer Sciences describe
their experiment in a paper published Oct. 1 in the journal Optics
Letters. Co-authors of the paper include Hailin Wang at the University
of Oregon and Shun-Lien Chuang at the University of Illinois at
Urbana-Champaign.

Prior experiments in the past five years have demonstrated
that light beams can be slowed or accelerated through atomic vapor as
well as solid state crystal. Physicists at Harvard University have even
managed to stop light particles in their tracks for 10-20 microseconds
in rubidium gas.

The UC Berkeley-led team is the first to experimentally demonstrate slow light using thin layers of semiconductors.

"Semiconductors offer a critical bandwidth advantage over atomic
vapor and solid state crystal," said Chang-Hasnain, who is also
director of UC Berkeley's Center for Optoelectronic Nanostructured
Semiconductor Technologies. "Semiconductors have 1 million to 1 billion
times broader bandwidth capacity than atomic gas or crystal. This
brings real-world practicality to telecommunication and network
applications. Another advantage to using semiconductors is the
possibility of cost-effective integration into circuitry, specifically
photonic integrated circuits."

Currently, optical signals can zip along fiber at over 62,000
miles per second - until they reach one of the many junctions along the
way. At that point, the light signals are converted to slower moving
electronic data so they can be read by routers. The routers then direct
the data to the correct path before converting the signals back to
light. This optical-electronic-optical (OEO) conversion is incredibly
slow and is expensive to power, the researchers say, resulting in huge
bottlenecks and significant cost for the network.

"The routers are like traffic lights at motorway
intersections," said Chang-Hasnain. "With OEO conversion, it's as if at
every intersection, drivers would have to get out of their cars, speak
with a traffic control officer, and fill out different forms indicating
their destination before moving on. You would need to have traffic
control officers who can speak different languages to communicate with
all the different drivers. As a result, you end up with a huge traffic
jam at each intersection."

Pei-Cheng Ku, lead author of the paper and a UC Berkeley
post-doctoral researcher in electrical engineering at the time of the
study, compares controlling light's velocity to installing brakes on a
car. "Enabling drivers to adjust their speed is necessary so they can
avoid colliding into one another; it's the same with optical signals,"
he said. "This would eliminate the need for traffic control officers,
or electronic routers, by letting drivers stay in their cars."

"Right now, we are not taking full advantage of the 20
terahertz bandwidth that fiber can provide because of the limitations
of these OEO switching systems," said Ku. "If we did, we'd be able to
send 600 two-hour feature films in about 1 second."

Ku is careful to point out that the experiments do not violate
the laws of physics, which state that the speed of light is a physical
constant that cannot be changed. What can be, and has been, changed is
the speed with which a light wave's amplitude, also known as the
"envelope" of a wave, is transmitted.

All the "slow" and "fast" light experiments that have been
conducted to date exploit the way light is absorbed and dispersed when
it interacts with matter. In a technique called electromagnetically
induced transparency (EIT), a "control" laser beam is fired into atomic
vapor to interfere with the ability of the atoms to absorb light
particles. Researchers then shine a "signal" beam into the vapor, which
disperses the light pulses and dramatically slows them down. This is
measured by an increase in the refractive index as it passes through
the medium.

Other researchers, including the UC Berkeley-led team, have
used a second method to demonstrate slow light called coherent
population oscillation (PO). This involves simultaneously shining two
laser beams - a "pump" beam and a less intense "probe" beam - of
slightly different frequencies at a sample. The differing frequencies
create a rhythmic beating pattern that creates light-slowing
interference.

Semiconductor quantum wells were used as the sample in the UC
Berkeley-led experiments. The device consisted of a thin layer of
electron-confining gallium arsenide sandwiched between two relatively
thicker layers of aluminum gallium arsenide.

The researchers conducted the experiments at the ultra-low
temperature of 10 Kelvin, or minus 442 degrees Fahrenheit. They are
currently working on using EIT in semiconductors to slow light down at
room temperature.

Other researchers on the project are Forrest Sedgwick at UC
Berkeley, Phedon Palinginis and Tao Li at the University of Oregon, and
Shu-Wei Chang at the University of Illinois at Urbana-Champaign.

The Defense Advanced Research Projects Agency and the Army Research Office provided support for the project.

• Hypography Science Feed
• Science Forums Feed
• Science Blogs Feed