Abstract
<title>Abstract</title> <p>Antiferromagnets exhibit rapid spin dynamics in a net zero magnetic background which enables novel spintronic applications and interrogation of manybody quantum phenomena. The layered antiferromagnet Sr2IrO4 hosts an exotic spin one-half Mott insulating state with an electronic gap arising from on-site Coulomb repulsion and strong spin-orbit coupling. This makes Sr2IrO4 an interesting candidate to interrogate dynamical attributes of the magnetic order using ultrafast laser pulses. We investigate the magnetization dynamics of Sr2IrO4 following circularly polarized photoexcitation with below-gap mid-infrared (mid-IR – 9 μm) and above-gap near-infrared (near-IR – 1.3 μm) pulses. In both cases, we observe excitation of a zone-center coherent magnon mode featuring a 0.5 THz oscillation in the pump-induced Kerr-rotation signal. However, only below-gap excitation exhibits a helicity dependent response and linear (quadratic) scaling of the coherent magnon amplitude with excitation fluence (electric field). Moreover, below-gap excitation has a magnon generation efficiency that is at least two orders of magnitude greater in comparison to above-gap excitation. Our analysis shows that the helicity dependence and enhanced generation efficiency are consistent with a one-photon–two-magnon coupling mechanism for magnon generation. Thus, preferential spin-photon coupling without resonant photoexcitation of electrons permits extremely efficient magnon generation. Our results reveal new possibilities for ultrafast control of antiferromagnets.</p>