  Multiferroics permit the magnetic control of the electric polarization and
electric control of the magnetization. These static magnetoelectric (ME)
effects are of enormous interest: The ability to read and write a magnetic
state current-free by an electric voltage would provide a huge technological
advantage. Dynamic or optical ME effects are equally interesting because they
give rise to unidirectional light propagation as recently observed in
low-temperature multiferroics. This phenomenon, if realized at room
temperature, would allow the development of optical diodes which transmit
unpolarized light in one, but not in the opposite direction. Here, we report
strong unidirectional transmission in the room-temperature multiferroic
BiFeO$_3$ over the gigahertz--terahertz frequency range. Supporting theory
attributes the observed unidirectional transmission to the spin-current driven
dynamic ME effect. These findings are an important step toward the realization
of optical diodes, supplemented by the ability to switch the transmission
direction with a magnetic or electric field.
