IEEE Spectrum June, 2013 - 15

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The only problem is that silicon really "People have been working for decades to
doesn't want to emit light. That's because, make an efficient light emitter."
One drawback of Agarwal's technology,
unlike gallium arsenide and indium phosphide, it has an indirect bandgap, which Paniccia says, is that it emits visible light.
Silicon is opaque at visible wavelengths, and
means excited electrons and holes are
more likely to emit heat than light when Intel's work uses infrared light, at 1310 nm.
Agarwal counters that shorter-wavelength
they recombine.
To overcome that difficulty, Agarwal and visible light makes for waveguides much
his team turned to the plasmonic effect. closer in size to other components on a comLight striking the surface of a metal in puter chip. He adds that computer networks
contact with a dielectric, such as silicon
aren't the only aim of the experimental silidioxide, creates a surface plasmon, an
con emitters, which could shine in a varielectromagnetic oscillation that is strongly ety of applications that need visible light.
confined to the metal-dielectric interface.
In his previous position, as head of InAgarwal coated silicon nanowires with a tel's Photonics Technology Lab, Paniccia
5-nanometer layer of silicon dioxide and laid
developed an integrated silicon photonthem down on a glass substrate. He then ap- ics system that included a hybrid silicon/
plied a 100-nm layer of silver atop the wires. indium phosphide laser, a silicon modulaThe silver covered all of the wire, except the
tor, and a germanium photodetector. Intel
part that was in contact with the substrate, announced in January that it will soon be
forming a shape like the Greek letter omega. commercializing an optical link, based on
The interface between the silver and the sili- that system, that transmits data between
racks of servers at a rate of 100 gigabits per
con dioxide created a small region where
second. Engineering samples have already
light could be amplified.
been distributed to companies that will use
The team then shone a blue laser with
a wavelength of 458 nm on the nanowires. the technology, although no sales date has
Afterward, they measured light in the 470- to yet been announced.
Others have trodden the silicon path be700-nm range coming out of the silicon. The
plasmonic cavity had forced the excited elec- fore. Back in 2005, semiconductor company
trons to combine with the holes and emit STMicroelectronics, of Geneva, was touta photon before they could drop to their ing silicon light emitters that were doped
natural heat-producing, lower-energy state. with rare earth elements to produce light
The work is at an early stage and would
emissions. Company spokesman Michael
need a lot of effort to be made practical, Markowitz says the technology turned out
Agarwal says. He'd like to improve the quan- to be not as promising as hoped. "Many
tum efficiency of the system-the ratio of years ago, after a review of our market and
photons generated to photons taken in- technology priorities, we decided to stop
to 5 or 10 percent, up from the 1 percent this activity," he says.
Still, Paniccia says, silicon photonics is
he has demonstrated. In this initial experion its way. "I think we're at the beginning
ment he achieved diffuse light, but he wants
to build an actual laser. And as is generally of the revolution of this technology being
the case in laser development, he must re- commercialized," he says. He calls Intel's
hybrid laser "a good compromise" that
place energy input from another laser with
doesn't achieve holy grail status. "I would
an electric current.
Mario Paniccia, Intel fellow and general love to have a silicon laser. I encourage the
research in this area," he says. "I think it's
manager of silicon photonics operations
at Intel, calls the UPenn work "an interest- a very hard problem." And it's exactly what
ing new approach." But the holy grail of a Agarwal's group is working on.
silicon laser is still a long way off, he says. -neil savage

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Table of Contents for the Digital Edition of IEEE Spectrum June, 2013