Electrical diodes, which conduct electricity in one direction only, are commonplace. Now, Li Fan of Purdue University in Indiana and colleagues have made a diode for light. Naively, it might seem that such a device should be impossible because of the time-reversal invariance of matter–light interactions, but the trick here is to use nonlinear optics to make a complex, effective index-of-refraction. The unidirectionality then comes from coupling two different modes – in effect, the colour going in is not quite the same as the colour going out.

While this means that the "diode" cannot be used as a linear optical isolator, it is made from a pair of silicon rings and is compatible with standard CMOS technology. With a forward–backward transmission ratio of 28 dB, it could find a range of applications and herald a new approach to optical information processing.