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” microcircuits 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, 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 fibre-optic communications, she suggests.
At present, information that travels at the speed of light through optical fibre 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 conversion.
The all-optical switch is described in the October 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 travelling 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 nanometres, 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-nanometre 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 small space.
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 tuneable filters, the researchers suggest, for example to separate the many wavelengths of light in multiplexed optical fibre communications systems.