  In the cuprates, the opening of a d-wave superconducting (SC) gap is
accompanied by a redistribution of spectral weight at energies two orders of
magnitude larger than this gap. This indicates the importance to the pairing
mechanism of on-site electronic excitations, such as orbital transitions or
charge transfer excitations. Here, we resort to a three-pulse pump-probe scheme
to study the broadband non-equilibrium dielectric function in optimally-doped
$Bi_2Sr_2Ca_{0.92}Y_{0.08}Cu_2O_{8+{\delta}}$ and we identify two distinct
dynamical responses: i) a blueshift of the central energy of an interband
excitation peaked at 2 eV and ii) a change in spectral weight in the same
energy range. Photoexcitation with near-IR and mid-IR pulses, with photon
energies respectively above and below the SC gap, reveals that the transient
changes in the central energy are not modified by the onset of
superconductivity and do not depend on the pump photon energy. Conversely, the
spectral weight dynamics strongly depends on the pump photon energy and has a
discontinuity at the critical temperature. The picture that emerges is that,
while high-energy pulses excite quasiparticles in both nodal and thermally
inaccessible antinodal states, photoexcitation by low-energy pulses mostly
accelerates the condensate and creates excitations predominantly at the nodes
of the SC gap. These results, rationalized by kinetic equations for d-wave
superconducting gaps, indicate that dynamical control of the momentum-dependent
distribution of non-thermal quasiparticles may be achieved by the selective
tuning of the photoexcitation.
