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15 August 2011
In the August 5 issue of Science, researchers lead by the California Institute of Technology, Pasadena, describe a new technique to isolate light signals on a silicon chip.   The work is an important step towards creating an optical diode using material that is compatible with photonic integrated circuits.
An optical diode allows light to travel in one direction not the other. This type of component is needed in optical circuits to stabilize the optical components by preventing reflected light from interfering with their function.
Typically optical isolators are made out of magnetic or nonlinear materials, but such materials are bulky or difficult to integrate with the other functions required on a photonic chip.  Silicon would be the ideal choice for photonic integration, but it does not have the required nonlinear properties.
Now the Caltech team have designed and fabricated a “metallic-silicon waveguide system in which the optical potential is modulated along the length of the waveguide in such a way that the properties of the light are changed in the reverse direction”.  Although the 1.55 micron light isn’t completely blocked, the optical signals travelling in different directions don’t interact.
The system is compatible with conventional complementary metal-oxide-semiconductor (CMOS) processing, providing a route to chip-scale optical isolators for optical communications and computing. Although the work is just a proof-of-principle experiment, the researchers are already building an optical isolator that can be integrated onto a silicon chip.
By Pauline Rigby