Revealing distinct photoexcitation mechanisms of a coherent magnon in the van der Waals antiferromagnet NiPS₃

Magnon modes in antiferromagnets (AFMs) hold promise for applications in novel spintronic devices due to the lack of stray magnetic fields and the fast, terahertz (THz) oscillation frequencies. The ability to coherently stimulate such modes, and an understanding of the various mechanisms which allow for it, is therefore an important field of study. In our research, we utilize ultrafast optical pump-THz emission spectroscopy to investigate the coupling of high-energy electronic degrees of freedom (d-d transitions around 1 eV) to a low-energy (~5 meV), zone-center magnon mode in the van der Waals AFM NiPS₃. Tuning the parameters of an optical pump pulse such as the photon energy or polarization enables excitation of the magnon with variable properties. Specifically, whether we are on- or off-resonant with an on-site d­-­d transition, we observe distinct pump polarization and fluence dependences of the THz emission properties of the magnon mode. In the off-resonant case, the coherent magnon displays a cos(2θ) pump polarization dependence and a linear fluence dependence, which are characteristics of well-known magneto-optical effects. On resonance, we detect a saturation of the magnon mode at higher pump fluences and magnon emission that is independent of the pump polarization. Therefore, we find evidence of a new, dissipation-induced mechanism responsible for launching the coherent magnon.