Science 333, 729–733 (2011)

The ability to confine the direction of light propagation — an important function for optical communications — is usually provided by optical isolators. However, these devices are often in bulk form and do not suit chip-scale integration. Liang Feng and co-workers from Caltech and the University of California at San Diego in the USA and Nanjing University in China have now designed and fabricated a waveguide system that performs one-wave photonic mode conversion at the telecommunications wavelength of 1.55 μm. By combining mode conversion with a mode filter, non-reciprocal light propagation can be achieved. The metallic-silicon waveguide (200 nm thick and 800 nm wide) is compatible with CMOS-semiconductor processing, thus paving the way for chip-based isolators. The researchers report that although their isolator has an insertion loss of around 7 dB, this could be compensated for by incorporating optical gain into the device.