Ultrafast manipulation of multiferroic orders through nonlinear phonon excitation

Resonant phonon excitations drive coherent lattice vibrations to large amplitudes exceeding the harmonic regime, making anharmonic contributions become sizable. Nonlinear phononic rectification is a lattice analog to optical rectification, in which nonlinear coupling between phonon modes leads to a quasistatic distortion of the crystal structure, creating transient crystal geometries that are not accessible in equilibrium.

All-optical control of the magnetoelectric phase using femtosecond laser pulses is appealing for low-energy consumption spintronics. It has remained unexplored whether light can control multiple ferroic order parameters simultaneously in a single domain. We report the observation of ultrafast enhanced second-harmonic generation (SHG) in multiferroic BiFeO₃ by resonant excitation of a high-energy A₁ phonon with strong-field mid-infrared laser pulses. We find that both the ferroelectric and antiferromagnetic contributions to the SHG signal are enhanced on a sub-picosecond timescale. We perform phonon dynamics simulations by density functional theory calculations, which confirms the enhanced ferroelectric SHG signal. We conclude that coherent phonon excitation with ultrashort laser pulses concurrently controls ferroelectricity and antiferromagnetic order.

Recent discoveries of various light-induced long-lived synthetic topological polar structures in (PbTiO3)_m/(SrTiO3)_n superlattices highlight the power of structural and functional couplings in creating new ordered phases. Since the superlattices are on the verge of polarization reorganization, these materials may be exceptionally susceptible to dynamical lattice distortion under resonant phonon excitations. A synergy between materials design (superlattices) and light control (mid-infrared pumping) would create exciting opportunities to discover new quantum phases.