|
|
|
|
Controlling
Light with Light
On-chip Nanophotonic Structures School of Electrical and Computer Engineering , Cornell
University |
|
Interested in a graduate research position
in Nanophotonics?
Contact Michal Lipson
In
applications specify current affiliation and prior
research experience/publications if available.
Click on pictures to download high resolution versions
The
work is described in the October 28 issue of the journal Nature,
pp. 1081-1084, by Lipson
and members of the Cornell
Nanophotonics Group.
What have we demonstrated?
We have demonstrated a device that allows one low-powered beam of light to switch another on and off, on silicon, a key component for future photonic chips in which light replaces electrons for propagating signals. It is highly desirable to use silicon—the dominant material in the microelectronic industry—as the platform for these photonic chips.
Photonics on silicon has been suggested since the 1970’s, and previous light-beam switching devices on silicon have been demonstrated. However, they were excessively large (by microchip standards) or required that the beam of light that does the switching be very high-powered. It is only in the last few years, however, with the advancement of fabrication techniques that sub-micron-size photonic structures have been realized enabling some of the traditional limitations of silicon photonics to be overcome.
The approach developed confines the beam to be switched in a circular resonator, greatly reducing the footprint required on the chip and allowing a very small change in refractive index to shift the material from transparent to opaque.
Schematic description of an all-optical gate photonic structure. (A) In the off-state, the probe
signal, in red, does not penetrate the gate; (B)
in the on-state, a control signal opens the gate and allows the probe signal
to be transmitted.
The optical switch is based on a ring resonator, a device already familiar to photonics researchers. When a straight waveguide is placed tangent to a ring-shaped waveguide, photons traveling along the straight waveguide are diverted into the ring. When the photons' frequency matches that of the resonant frequency of the ring, which is determined by its circumference, they travel around it many times. For the reported experiments, we created a ring ten micrometers in diameter with a resonance wavelength of 1555.5 nanometers, in the near infrared.
To
turn the switch “off,” a second beam of light with a wavelength in the same
spectral range is sent through the system. This wavelength 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 of the silicon
and consequently shifts the resonant frequency of the ring enough that it
will no longer resonate with the 1555.5 nanometer signal. The process can
theoretically take place in a few tens of picoseconds.
A
similar effect can be used in a straight waveguide, but requires fairly long
distances. Because light travels many times around the ring the scattering
effect is enhanced and the signal can be controlled in a very small space.
What are the applications
of this device?
These structures
will find their first application in routing devices for fiber-optic communications.
At present, information that travels at the speed of light through optical
fibers must be converted at the end into electrical signals that are processed
on conventional electronic chips. These electrical signals can in turn be
converted back into optical signals for re-transmission, which in the end
makes this an extremely slow process. The all-optical switch enables routing
signals without the need of conversion to electronics.
Related projects in our lab
Related publications
Frequently asked questions (FAQ)
What the press and popular media say
Supported in part by DARPA
/ MTO
US Defense Advanced Research Projects Administration
| Copyright
(c) 2004 |
Comments? |
Updated
10/19/2004
|
