  The interaction of a single-cycle THz electric field with the topological
insulator $\mathrm{MnBi}_2\mathrm{Te}_4$ triggers strongly anharmonic lattice
dynamics, promoting fully coherent energy transfer between the otherwise
non-interacting Raman-active $E_g$ and infrared (IR)-active $E_u$ phononic
modes. Two-dimensional (2D) THz spectroscopy combined with modeling based on
the classical equations of motion and symmetry analysis reveals the multi-stage
process underlying the excitation of the Raman-active $E_g$ phonon. In this
process, the THz electric field first prepares a coherent IR-active $E_u$
phononic state and subsequently interacts with this state to efficiently excite
the $E_g$ phonon.
