Giant coupling of second-harmonic generation to a multiferroic polarization

Spectroscopy reveals a giant coupling of optical second-harmonic generation to the spontaneous polarization in compounds with magnetically driven ferroelectricity. In TbMn2O5 the symmetry reduction by the ferroelectric order is detected with a sensitivity four orders of magnitude above the resolution limit of 10-100 pC/cm(2). This points to an electronic rather than ionic origin of the spontaneous polarization in joint-order-parameter multiferroics. Hence, our work constitutes an inaugural experimental statement on the controversial theoretical discussion on the nature of the multiferroic polarization.