Photonic crystals are materials with bandgaps, which do not allow certain wavelengths of light to pass. Just as bandgaps in semiconductors forbid electrons to have certain energies, is it possible to construct a photonic analogue of a diode?

Zheng Wang of MIT and colleagues there and at Yale University have done just this. They constructed an array of magneto-optical ferrite rods placed in a magnetic field in such a way that microwaves passing in one direction were not allowed to propagate, while those moving the other way went through unimpeded, with a difference of 50 dB in the two directions.

Apart from the novelty of the physics involved, the work also suggests the possibility of novel optical fibres with hollow cores surrounded by materials of the kind described above. Having no material inside, they could allow very long-range transmission without repeaters.