  An electronic diode is a nonlinear semiconductor circuit component that
allows conduction of electrical current in one direction only. A component with
similar functionality for electromagnetic waves, an electromagnetic isolator,
is based on the Faraday effect of the polarization state rotation and is also a
key component of optical and microwave systems. Here we demonstrate a chiral
electromagnetic diode, which is a direct analogue of an electronic diode: its
functionality is underpinned by an extraordinary strong nonlinear wave
propagation effect in the same way as electronic diode function is provided by
a nonlinear current characteristic of a semiconductor junction. The effect
exploited in this new electromagnetic diode is an intensity-dependent
polarization change in an artificial chiral metamolecule. This microwave effect
exceeds a similar optical effect previously observed in natural crystals by
more than 12 orders of magnitude and a direction-dependent transmission that
differing by a factor of 65.
