To prevent microwaves passing through it from reflecting backward, a new ‘metamaterial’ uses antennas of alternating orientations (top) that are connected by amplifier circuits (bottom). Photo: Zheng Wang |
Computer
chips that use light to move data would be much more energy efficient
and possibly even faster than today’s chips, which use electricity. One
of the difficulties in realizing them, however, is that light moving
through a “waveguide”—unlike electrons moving through a wire—can reflect
backward, interfering with subsequent transmissions and even disrupting
the operation of the laser that emitted it.
Today’s
optical networks keep light from reflecting backward with devices
called isolators, which are made from exotic materials like yttrium
indium garnet and work only when a magnetic field is applied to them,
which makes them bulky. But since isolators absorb light
particles—photons—to prevent them from scattering backward, they also
diminish the strength of forward-moving optical signals.
In this week’s Proceedings of the National Academy of Sciences,
researchers at MIT, Zhejiang University in China, and the University of
Texas at Austin describe a new “metamaterial” that keeps photons moving
in only one direction, rechanneling the stragglers rather than simply
absorbing them. Although the prototype is large, it doesn’t require the
application of a magnetic field, so it could, in principle, yield
optical components much smaller than today’s isolators. Moreover,
building a chip-scale version of the metamaterial would require no
materials more exotic than the metals already used in microprocessors,
reducing manufacturing costs.
With
isolators, “you may not have reflection, but you lose light as light
propagates in your structure,” says Zheng Wang, who led the research as a
postdoc and research scientist at MIT and is now an assistant professor
of computer and electrical engineering at the University of Texas.
“Which is a big deal, because one of the reasons we don’t have
large-scale integrated optical devices is that loss limits how many
devices we can integrate in the system.”
Wang’s
co-authors on the paper are MIT physics professors Marin Soljacic and
John Joannopoulos; Lixin Ran and Zhiyu Wang of Zhejiang University; and
MIT graduate students Jingyu Wang, Bin Zhang and Jiangtao Huangfu.
What
gives the researchers’ new material its light-herding properties are
rows of embedded metal antennas that look rather like small, twin-blade
propellers of alternating orientations, vertical and horizontal. Each
antenna is connected by an electrical circuit to an antenna of the
opposite orientation on the bottom surface of the material; the
direction of current flow through the electrical circuits determines the
direction in which electromagnetic waves will propagate.
In
the prototype, the antennas are embedded in a pair of circuit boards
about an inch apart, but in a chip, they could just as easily be
embedded in silicon. Indeed, miniaturizing the antennas is not the chief
obstacle to getting the metamaterial to work at visible-light or even
near-infrared frequencies. The operating frequency is also limited by
the switching speed of a transistor in the electrical circuit, and no
known transistor design yields a switching speed high enough for visible
light.
“We probably need to use some nonlinear optics,” Wang says. “That’s something we’re still working on.”
“It’s
pretty exciting work,” says Shanhui Fan, an associate professor of
electrical engineering at Stanford University. “It’s addressing an
important problem.” While researchers are investigating a number of
different approaches to chip-scale waveguides, an optical version of the
new metamaterial could be particularly useful in the manufacture of the
on-chip devices that control optical signals. Though the
back-reflection in such devices can be mild, it compounds when a number
of them are used in sequence. “That turned out to be quite an issue if
you need to cascade many of these devices together,” Fan says.
Fan’s
own group is working on an approach to preventing back-scattering that
works at optical frequencies. “Scaling their approach up to a higher
frequency would be a very exciting direction,” Fan says, but he adds, “I
imagine that there will be important applications in every frequency
range.”
Gyrotropic response in the absence of a bias field
Source: MIT
