Observation of angular momentum transfer among crystal lattice modes

Transfer of energy and linear momentum between lattice vibrations via anharmonic coupling is an important concept in solid-state physics. However, it remained difficult to directly observe how angular momentum is exchanged and conserved among lattice modes, even though these processes are thought to play an important role in achieving magnetization equilibrium and in spin relaxation effects like the Einstein-de Haas effect. Here we demonstrate and coherently control angular momentum transfer between two lattice modes using the inverse process of anharmonic decay. The observed rotational phonon-phonon Umklapp scattering enforces the conservation of quantized crystal angular momentum, as dictated by the discrete rotational symmetry of the crystal. We thereby experimentally confirm the fundamental analogy between linear and angular momentum conservation in solids. Moreover, we establish axial nonlinear phononics as a promising handle for the ultrafast control of material properties.