With a bit of leverage, Cornell
researchers have used a very tiny beam of light with as little as 1 milliwatt
of power to move a silicon structure up to 12 nanometers. That's enough to completely
switch the optical properties of the structure from opaque to transparent, they
reported.
 | | Scanning electron micrograph of two thin, flat rings of silicon nitride, each 190 nanometers thick and mounted a millionth of a meter apart. Light is fed into the ring resonators from the straight waveguide at the right. Under the right conditions optical forces between the two rings are enough to bend the thin spokes and pull the rings toward one another, changing their resonances enough to act as an optical switch. Cornell Nanophotonics Group |
The technology could have applications in the design of micro-electromechanical
systems (MEMS) -- nanoscale devices with moving parts -- and micro-optomechanical
systems (MOMS) which combine moving parts with photonic circuits, said Michal
Lipson, associate professor of electrical and computer engineering.
The research by postdoctoral researcher Gustavo Wiederhecker, Long Chen Ph.D.
'09, Alexander Gondarenko Ph.D. '10 and Lipson appears in the online edition
of the journal Nature and will appear in a forthcoming print edition.
Light can be thought of as a stream of particles that can exert a force on
whatever they strike. The sun doesn't knock you off your feet because the force
is very small, but at the nanoscale it can be significant. "The challenge
is that large optical forces are required to change the geometry of photonic
structures," Lipson explained.
But the researchers were able to reduce the force required by creating two
ring resonators -- circular waveguides whose circumference is matched to a multiple
of the wavelength of the light used -- and exploiting the coupling between beams
of light traveling through the two rings.
A beam of light consists of oscillating electric and magnetic fields, and these
fields can pull in nearby objects, a microscopic equivalent of the way static
electricity on clothes attracts lint. This phenomenon is exploited in "optical
tweezers" used by physicists to trap tiny objects. The forces tend to pull
anything at the edge of the beam toward the center.
When light travels through a waveguide whose cross-section is smaller than
its wavelength some of the light spills over, and with it the attractive force.
So parallel waveguides close together, each carrying a light beam, are drawn
even closer, rather like two streams of rainwater on a windowpane that touch
and are pulled together by surface tension.
The researchers created a structure consisting of two thin, flat silicon nitride
rings about 30 microns (millionths of a meter) in diameter mounted one above
the other and connected to a pedestal by thin spokes. Think of two bicycle wheels
on a vertical shaft, but each with only four thin, flexible spokes. The ring
waveguides are three microns wide and 190 nanometers (nm -- billionths of a
meter) thick, and the rings are spaced 1 micron apart.
When light at a resonant frequency of the rings, in this case infrared light
at 1533.5 nm, is fed into the rings, the force between the rings is enough to
deform the rings by up to 12 nm, which the researchers showed was enough to
change other resonances and switch other light beams traveling through the rings
on and off. When light in both rings is in phase -- the peaks and valleys of
the wave match -- the two rings are pulled together. When it is out of phase
they are repelled. The latter phenomenon might be useful in MEMS, where an ongoing
problem is that silicon parts tend to stick together, Lipson said.
An application in photonic circuits might be to create a tunable filter to
pass one particular optical wavelength, Wiederhecker suggested.
The work is supported by the National Science Foundation (NSF) and the Cornell
Center for Nanoscale Systems. Devices were fabricated at the Cornell Nanoscale
Science and Technology Facility, also supported by NSF.
Posted November 16th, 2009
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