Fig. S1.

(A) Schematic illustration of the experimental setup. Here, SGGG is a nonmagnetic substrate (Materials and Methods). (B) Pump intensity dependence of the transient Faraday rotation when the pump spot overlaps with the probe spot. Pump polarization was changed between right- and left-circular ($+\sigma$ and $-\sigma$, respectively) at each pump intensity with the wavelength of 1,300 nm. (C and D) Probe position dependence of the transient Faraday rotation at the pump intensity of 45 nJ (C) and 100 nJ (D). The Inset of C shows the distance d between pump and probe spot, which is measured 45° off the x axis. For C and D, data were offset, proportionally to the distance d, for clarity.