  We compute the one-particle spectral function and the optical conductivity
for the 2-d Hubbard model on a square lattice. The computational method is
cellular dynamical mean-field theory (CDMFT) in which a 4-site Hubbard
plaquette is embedded in a self-consistent bath. We obtain a `kink' feature in
the dispersion of the spectral function and a mid-infrared (mid-IR) absorption
peak in the optical conductivity, consistent with experimental data. Of the 256
plaquette states, only a single state which has d$_{x^2-y^2}$ symmetry
contributes to the mid-IR, thereby suggesting a direct link with the pseudogap.
Local correlations between doubly and singly occupied sites which lower the
kinetic energy of a hole are the efficient cause of this effect.
