Fig. 3: Evolution of the scanning ultrafast electron microscopy (SUEM) contrasts at high optical fluences.

The evolution of the SUEM contrast in the center dark region and the outer bright region at different optical pump fluences is shown in (a) and (b). Normalized versions are shown in (c) and (d). While the relaxation dynamics in the outer bright region are independent of the optical pump fluence, a sharp transition near 250 μJ/cm2 is observed in the center region, signaling a photo-induced insulator-to-metal transition. Significant noise at higher optical fluences is due to reduced overall SUEM contrast.