Extended Data Fig. 4: Investigation of the excitation mechanism from the pump polarization dependence.

Panel a) shows the residual Kerr rotation for a variety of incident pump polarizations and panel b) shows the tracked amplitude of the oscillations in a) as function of the pump polarization. The oscillation amplitude depends strongly on the incident polarization, with the maximum amplitude occurring at diagonal polarization between the crystallographic a- and b axis. Therefore, the excitation mechanism most strongly matches to the Inverse Cotton-Mouton Effect symmetry. Thus, the spin waves are launched through non-thermal effects, in the form of Impulsive Stimulated Raman (or Brillouin) Scattering.