  Time resolved measurements provide a new way to disentangle complex
interactions in quantum materials due to their different timescales. We used
pump-probe Raman scattering to investigate the apical oxygen vibration in
YBa$_2$Cu$_3$O$_{6+x}$ under nonequilibrium conditions. Time-dependence of the
phonon population demonstrated strong electron-phonon coupling. Most
importantly, the phonon shifts to a higher energy due to transient smearing of
the Fermi surface in a remarkable agreement with diagrammatic theory. We also
discuss new insights into photoinduced superconductivity reported at lower
doping that follow from these results.
