Two-Dimensional Terahertz Spectroscopy of Nonlinear Phononics in the Topological Insulator MnBi2Te4

The interaction of a single-cycle terahertz electric field with the topological insulator MnBi2Te4 triggers strongly anharmonic lattice dynamics, promoting fully coherent energy transfer between the otherwise noninteracting Raman-active Eg and infrared (IR)-active Eu phononic modes. Two-dimensional terahertz spectroscopy combined with modeling based on the classical equations of motion and symmetry analysis reveals the multistage process underlying the excitation of the Raman-active Eg phonon. In this nonlinear combined photophononic process, the terahertz electric field first prepares a coherent IR-active Eu phononic state and subsequently interacts with this state to efficiently excite the Eg phonon.