Selective probe of coherent polar phonon and quasiferromagnetic resonance modes in multiferroic GdFeO3

Using time-resolved second harmonic generation (TRSHG) and optical transmission, we demonstrate simultaneous optical access to both magnetic and ferroelectric order parameters in a magnetoelectric multiferroic GdFeO3 below the transition temperature 2.2 K. In magnetoelectric phase, the pump pulses impulsively generate polar phonon modes of B2 and A1 symmetry and a quasiferromagnetic resonance mode, creating coherent lattice and spin motions. While collective motions associated with the ferroelectric and antiferromagnetic ordering are simultaneously excited, only the polar motion is detected in TRSHG, resulting in a direct subpicosecond excitation of the improper ferroelectric order. Our result also experimentally indicates that the soft mode in improper ferroelectrics is IR inactive above the phase transition, unlike proper ferroelectrics. Furthermore, we investigate the crystal polar symmetry and potential coupling between the polar phonons and the quasiantiferromagnetic resonance mode, which can shed light on the studies of subpicosecond magnetoelectric coupling based on collective quasiparticle excitations.