Terahertz control of linear and nonlinear Magno-phononics

Coherent manipulation of magnetism through the lattice provides opportunities for controlling spintronic functionalities on the ultrafast timescale. Such nonthermal control typically involves nonlinear excitation of Raman-active phonons which are coupled to the magnetic order. Linear excitation, in contrast, holds potential for more efficient and selective modulation of magnetic properties. However, since the linear excitation of Raman-active phonons is conventionally considered forbidden in inversion symmetric quantum materials, the simultaneous linear and nonlinear excitation of a collective mode involving lattice component has remained elusive. Here, we harness strong coupling between magnons and Raman-active phonons to achieve both linear and quadratic excitation regimes of magnon-polarons, magnon-phonon hybrid quasiparticles. We demonstrate this by driving magnon-polarons with an intense terahertz pulse in the van der Waals antiferromagnet FePS₃. Such excitation behavior enables a unique way to coherently control the amplitude of magnon-polaron oscillations by tuning the terahertz field strength and its polarization. The polarimetry of the resulting coherent oscillation amplitude breaks the crystallographic C₂ symmetry due to strong interference between different excitation channels. Our findings unlock a wide range of possibilities to manipulate material properties, including modulation of exchange interactions by phonon-Floquet engineering.