  A new approach to all-optical detection and control of the coupling between
electric and magnetic order on ultrafast timescales is achieved using
time-resolved second harmonic generation (SHG) to study a ferroelectric
(FE)/ferromagnet (FM) oxide heterostructure. We use femtosecond optical pulses
to modify the spin alignment in a
Ba$_{0.1}$Sr$_{0.9}$TiO$_{3}$(BSTO)/La$_{0.7}$Ca$_{0.3}$MnO$_{3}$ (LCMO)
heterostructure and selectively probe the ferroelectric response using SHG. In
this heterostructure, the pump pulses photoexcite non-equilibrium
quasiparticles in LCMO, which rapidly interact with phonons before undergoing
spin-lattice relaxation on a timescale of tens of picoseconds. This reduces the
spin-spin interactions in LCMO, applying stress on BSTO through
magnetostriction. This then modifies the FE polarization through the
piezoelectric effect, on a timescale much faster than laser-induced heat
diffusion from LCMO to BSTO. We have thus demonstrated an ultrafast indirect
magnetoelectric effect in a FE/FM heterostructure mediated through elastic
coupling, with a timescale primarily governed by spin-lattice relaxation in the
FM layer.
