those two edges, $ \sigma(\delta t , \omega ) $ shows spectral oscillations in $\omega$ with a frequency $\delta \omega$ that is inversely proportional to the delay time, i.e., $\delta\omega =  ( 2 \pi ) / |\delta t| $.[@Papen07]

Interestingly, we find that the pump-probe response $ \sigma(\delta t , \omega ) $ also exhibits a rich oscillatory behavior in the delay-time dependence, with multiple frequencies that depend on the absorption energy $\hbar \omega$; see Figs. \[FigM3\](a) and \[FigM3\](b). This is revealed most clearly in Fig. \[FigM3\](b), which shows the Fourier transformed pump-probe signal in two-dimensional frequency space, i.e., $\textrm{Re} ( \sigma  )  $ as a function of Fourier frequency $\omega_{\delta t}$ of the delay time and absorption frequency $\omega$. We observe that for the absorption energy $\hbar \omega$ within the interval $0 < \hbar \omega < \hbar \omega_{\Delta_{\infty}}$, $\textrm{Re} [ \sigma(\delta t , \omega )]$ oscillates in $\delta t$ with the frequency $\omega_{\Delta (t_i)}$. The oscillatory behavior of $\textrm{Re} [ \sigma(\delta t , \omega )]$ in the interval $\hbar \omega_{\Delta_{\infty}} < \hbar \omega <  \hbar \omega_{\Delta (t_i)}$ is even more intriguing, as it shows signatures of how the gap decreases while the pump pulse acts on the sample \[cf. 