  One of the pivotal questions in the physics of high-temperature
superconductors is whether the low-energy dynamics of the charge carriers is
mediated by bosons with a characteristic timescale. This issue has remained
elusive since electronic correlations are expected to dramatically speed up the
electron-boson scattering processes, confining them to the very femtosecond
timescale that is hard to access even with state-of-the-art ultrafast
techniques. Here we simultaneously push the time resolution and the frequency
range of transient reflectivity measurements up to an unprecedented level that
enables us to directly observe the 16 fs build-up of the effective
electron-boson interaction in hole-doped copper oxides. This extremely fast
timescale is in agreement with numerical calculations based on the t-J model
and the repulsive Hubbard model, in which the relaxation of the photo-excited
charges is achieved via inelastic scattering with short-range antiferromagnetic
excitations.
