One way traffic
The diode is a basic two-terminal device which allows electrons to flow in one direction but not in the opposite direction. One would be hard pressed to find a modern electronic gadget that does not employ this elemantary component in its circuitry. It is envisioned that photonics, or computation using light zooming around in waveguides on an integrated chip, would soon replace electronics offering improved speed and lower power consumption. An obvious goal then, is to develop the optical version of the diode. Now, researchers at the California Institute of Technology and the University of California, San Diego have demonstrated such “non-reciprocal” light propagation in a waveguide on a silicon chip.
Traditionally, it has been very difficult to get photons to obey the rules of a one-way street. Exotic magneto-optic materials and large magnetic fields are required to exploit the “Faraday effect” of light and make it behave differently while travelling in different directions in a bulk medium. However, doing the same on nanoscale dimensions on a tiny chip has proved to be far from trivial. The recent work achieves non-reciprocal behaviour using conventional materials and standard nanofabrication technology used to manufacture computer chips today.
The key insight is to engineer a “complex” (here referring to the mathematical term) optical potential along the waveguide which couples to light traveling only in one direction. This is achieved by having periodic “bumps” of silicon and germanium/chromium along the waveguide, which offer a different array of scattering surfaces to light travelling in the forward direction as compared to the backward direction. For a loose analogy, imagine riding an escalator – it is easy to walk in the same direction as the escalator is moving, but much harder to move against its motion. Light travelling in the forward direction encounter the silicon bumps just before the germanium/chromium ones, and the same pattern repeats after another small propagation distance. On the contrary, for backward propagating light the germanium/chromium bumps appear just before the silicon ones, and again the same pattern repeats after another small propagation distance.
The net effect is that the transverse spatial distribution of the light or the “optical mode” is preserved for one propagation direction and is modified for the opposite direction, and one can filter out a particular mode at the end of the waveguide.
Feng, L., Ayache, M., Huang, J., Xu, Y., Lu, M., Chen, Y., Fainman, Y., & Scherer, A. (2011). Nonreciprocal Light Propagation in a Silicon Photonic Circuit Science, 333 (6043), 729-733 DOI: 10.1126/science.1206038