  Ultrafast spectroscopy is an emerging technique with great promise in the
study of quantum materials, as it makes it possible to track similarities and
correlations that are not evident near equilibrium. Thus far, however, the way
in which these processes modify the electron self-energy---a fundamental
quantity describing many-body interactions in a material---has been little
discussed. Here we use time- and angle-resolved photoemission to directly
measure the self-energy's ultrafast response to near-infrared photoexcitation
in high-temperature cuprate superconductor. Below the superconductor's critical
temperature, ultrafast excitations trigger a synchronous decrease of electron
self-energy and superconducting gap, culminating in a saturation in the
weakening of electron-boson coupling when the superconducting gap is fully
quenched. In contrast, electron-boson coupling is unresponsive to ultrafast
excitations above the superconductor's critical temperature and in the metallic
state of a related material. These findings open a new pathway for studying
transient self-energy and correlation effects in solids.
