Switching one light beam with another, Cornell provides a key component for photonic chips...
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Switching one light beam with another, Cornell provides a key component for
EMBARGOED FOR RELEASE: WEDNESDAY, OCT. 27, 2004, 1 P.M. EDT
Contact: Bill Steele
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Scanning electron microscope photo of a ring resonator coupled to two
straight waveguides. By making the resonator either transparent or opaque to
a particular wavelength of light, a photonic circuit could control whether
or not the light passes from one straight waveguide to the other. Click on
the image for a high-resolution version (1280 x 960 pixels, 1236K)
ITHACA, N.Y. -- Cornell University researchers have demonstrated for the
first time a device that allows one low-powered beam of light to switch
another on and off on silicon, a key component for future "photonic"
microchips in which light replaces electrons.
Photonics on silicon has been suggested since the 1970s, and previous
light-beam switching devices on silicon have been demonstrated, but they
were excessively large (by microchip standards) or have required that the
beam of light that does the switching be very high-powered. The approach
developed by Michal Lipson, Cornell assistant professor of electrical and
computer engineering, confines the beam to be switched in a circular
resonator, greatly reducing the space required and allowing a very small
change in refractive index to shift the material from transparent to opaque.
The advancement of nanoscale fabrication techniques in just the past few
years has made it possible to overcome some of the traditional limitations
of silicon photonics, Lipson said. Photonic circuits will find their first
application in routing devices for fiber-optic communications, she suggests.
At present, information that travels at the speed of light through optical
fiber must be converted at the end into electrical signals that are
processed on conventional electronic chips, then in many cases converted
back into optical signals for retransmission, an extremely slow process. The
all-optical switch makes it possible to route these signals without
The all-optical switch is described in the Oct. 28 issue of the journal
Nature by Lipson and members of the Cornell Nanophotonics Research Group,
which she directs. The researchers used the facilities of the Cornell
NanoScale Facility to manufacture the devices on silicon chips. "It is
highly desirable to use silicon -- the dominant material in the
microelectronic industry -- as the platform for these photonic chips," they
said in their paper. The group already has developed other components for
silicon photonic chips, including straight and curved waveguides. One of the
key components needed, however, is a way for one optical signal to switch
another on or off.
Lipson's optical switch is based on a ring resonator, a device already
familiar to photonics researchers. When a ring-shaped waveguide is placed
tangent to a straight one, photons traveling along the straight waveguide
are diverted into the ring and travel around it many times, but only if they
match the resonant frequency of the ring, which is determined by its
circumference. For the reported experiments, the researchers created a ring
10 micrometers in diameter with a resonance wavelength of 1,555.5
nanometers, in the near infrared.
To turn the switch off, they pumped a second beam of light in the same
wavelength range through the system. This light is absorbed by the silicon
through a process known as two-photon absorption, creating many free
electrons and "holes" (positively charged regions) in the material. This
changes the refractive index and shifts the resonant frequency of the ring
far enough that it will no longer resonate with the 1,555.5-nanometer
signal. The process can theoretically take place in a few tens of
picoseconds, the researchers said.
A similar effect can be used in a straight waveguide, but it requires a
fairly long distance. Because light travels many times around the ring, the
scattering effect is enhanced and the signal can be controlled in a very
For routing applications, Lipson said, a ring resonator coupled to two
waveguides could be used. The second waveguide would receive a signal only
when the resonator is switched on. She noted that there is very little loss
of light in the ring, meaning that light coming into a routing device could
be "recycled" and sent on its way with no additional amplification needed.
Ring resonators also could be used as tunable filters, the researchers
suggest, for example to separate the many wavelengths of light in
multiplexed optical fiber communications systems.
The Nature paper is titled "All-optical switch on silicon: Controlling light
with light on chip." Co-authors are Vilson Almeida, a former Cornell
graduate student now in the Institute for Advanced Studies in the Technical
Center of the Brazilian Air Force ; Carlos Barrios, former Cornell
postdoctoral researcher and now a scientist in the Nanophotonics Technology
Centre , Universidad PolitÚnica de Valencia, Spain; and Roberto Panepucci,
former Cornell research associate now an assistant professor at Florida
Previous work on nanoscale optical waveguides and photonic coupling is
described in a paper, "Overcoming the limitations of microelectronics using
Si nanophotonics: solving the coupling, modulation and switching
challenges," published in the Institute of Physics journal Nanotechnology ,
Aug. 2, 2004.
Related World Wide Web sites: The following sites provide additional
information on this news release.
oThe Cornell nanophotonics group: [Only registered users see links. ]
oPrevious Cornell News Service story on photonic microchips: