  Atomically thin van der Waals magnetic materials have not only provided a
fertile playground to explore basic physics in the two-dimensional (2D) limit
but also created vast opportunities for novel ultrafast functional devices.
Here we systematically investigate ultrafast magnetization dynamics and spin
wave dynamics in few-layer topological antiferromagnetic MnBi2Te4 crystals as a
function of layer number, temperature, and magnetic field. We find
laser-induced (de)magnetization processes can be used to accurately track the
distinct magnetic states in different magnetic field regimes, including showing
clear odd-even layer number effects. In addition, strongly field-dependent
antiferromagnetic magnon modes with tens of gigahertz frequencies are optically
generated and directly observed in the time domain. Remarkably, we find that
magnetization and magnon dynamics can be observed in not only the time-resolved
magneto-optical Kerr effect but also the time resolved reflectivity, indicating
strong correlation between the magnetic state and electronic structure. These
measurements present the first comprehensive overview of ultrafast spin
dynamics in this novel 2D antiferromagnet, paving the way for potential
applications in 2D antiferromagnetic spintronics and magnonics as well as
further studies of ultrafast control of both magnetization and topological
quantum states.
