Extended Data Fig. 5: Importance of nonlinearly excited low energy modes and THz field strength dependence of the metastable signal.

We repeat the same experiment with single-cycle THz excitation pulses generated from LiNbO₃ by tilted-pulse-front technique. The THz spectrum is given in a (blue), that is below the lowest phonon observed in FePS₃. In b we compare signals induced by two different THz pulses generated by organic crystal (BNA) and LiNbO₃ (see Methods) at T = 118 K, and in the case of LiNbO₃ we did not observe any long-lived signal. c. The amplitude of the metastable signal is quadratic in THz field amplitude. This is in agreement with the expected linear dependence of the induced M on the phonon displacement Q₂, whereas Q₂ has an excitation pathway quadratic on E_THz. Hence M is expected to scale quadratically with E_THz.