Fig. 4.

Fluence dependence. (A) Pump-induced suppression ($1-I(\mathrm{\Delta }t)/I_0$) of the O $K$ resonance $Q=(001)$ tr-RXS intensity at short time delay as a function of fluence. The 20 K signal at the planar O $K$ resonance comprises both lattice and electronic contributions, while the apical resonance is sensitive only to structural anisotropy. The signal at the planar O $K$ resonance is also shown at $T=100\text{K}>T_{\text{CDW}}$, where its fluence dependence is qualitatively similar to that observed at the apical O $K$ resonance. The shaded red line is a linear fit to the apical O $K$ resonance data. (B) Fluence-dependent suppression of the electronic nematic signal at $+0.5$ ps (near maximum suppression). The fluence-dependent response of the lattice is captured by a linear fit to the 20 K apical O data plotted in red in (A) and then subtracted from fluence scans collected at the planar O $K$ resonance inside the CDW phase.