Fig. 1: THz-2DCS of magnonic multiplication in Sm_0.4Er_0.6FeO₃.

a Experimental schematics. Two phase-locked THz pulses with inter-pulse delay τ are focused onto the sample. The magnetic fields of this THz pulse-pair are polarized parallel to the sample c-axis to excite the quasi-AFM mode. The coherent nonlinear emission signals are detected through electro-optic sampling. b The static THz transmission measurement of the sample, which describes the linear magnonic response, shows a peak at the magnon mode energy ~ 2.5 meV ( ~ 0.6 THz). c and d, Typical four channel THz signals measured by THz-2DCS at a fixed delay time τ = 4.8 ps. Two individual THz pulses used here, E₁ (red line) and E₂ (blue line), are shown in c. The coherent emission signal when both pulses are present, E₁₂ (light blue line), as well as the extracted nonlinear signal E_NL (black line), obtained by subtracting the two individual pulse contributions from E₁₂ (zoom in 100 × ), are plotted in d. The Fourier Transform of E_NL is performed for the time window after the pulse indicated by the two dotted red lines. e The E_NL spectrum shows up to six--magnon multiplication (dashed lines) for THz magnetic fields parallel to the c-axis (black solid line). The magnon nonlinear peaks are absent for THz magnetic fields perpendicular to the c-axis (blue solid line).