Femtosecond activation of magnetoelectricity

In magnetoelectric and multiferroic materials, the magnetic degree of freedom can be controlled by electric field, and vice versa. A significant amount of research has been devoted to exploiting this effect for magnetoelectric data storage and manipulation devices driven by d.c. electric fields(1-4). Aiming at ever-faster schemes of magnetoelectric manipulation, a promising alternative approach offers similar control on a femtosecond timescale, relying on laser pulses(4-6) to control both the charge(7,8) and the magnetic(9,10) order of solids. Here we photo-induce magnetoelectricity and multiferroicity in CuB2O4 on a sub-picosecond timescale. This process is triggered by the resonant optical generation of the highestenergy magnetic excitations-magnons with wavevectors near the edges of the Brillouin zone. The most striking consequence of the photo-excitation is that the absorption of light becomes non-reciprocal, which means that the material exhibits a different transparency for two opposite directions of propagation of light. The photo-induced magnetoelectricity does not show any decay on the picosecond timescale. Our findings uncover a path for ultrafast manipulations of the intrinsic coupling between charges and spins in multiferroics(4), which may reveal unexplored magnetic configurations and unravel new functionalities in terms of femtosecond optical control of magnetism.