Nonlinear Phononics in the Two-Dimensional Magnet CrSBr

Magnetism in two-dimensional (2D) van der Waals (vdW) crystals offers promising directions for low-dimensional physics and devices. CrSBr is a novel 2D vdW semiconductor that has received intense research interest due to its strain-dependent, low-dimensional magnetic properties. Mega-electron volt (MeV) ultrafast electron diffraction (UED) revealed that fs optical pump pulses excited non-equilibrium atomic displacements observed as coherent acoustic phonons (CAPs). Analyzing Bragg peak intensities allowed the determination of GHz-frequency acoustic disturbances propagating parallel to the surface normal and anisotropic phonon dynamics in the a-b plane. The temperature and crystallographic dependence of the phonons suggest that antiferromagnetic ordering influences the phonon oscillatory behavior. We observed subharmonic phonon frequencies which suggest a nonlinear oscillatory coupling between the laser-induced pump phonons and the secondary, subharmonic phonons. Thus, CrSBr was found to serve as a nonlinear phononic frequency conversion material. Our work provides a foundation for exploring how fs light pulses can influence phonon dynamics in materials with strong spin-lattice coupling. These results suggest that CAPs can be tuned by the sample dimension to match the magnon frequencies and show the promise of CrSBr for use in optical-to-microwave transducers and phononic devices.