  Femto-second optical pair breaking spectroscopy is performed on optimally
doped YBa2Cu3O7-delta near 1.5eV. Dramatic photon energy dependence with a
sharp triplet fine structure is seen for the first time. The peak separations
are attributable to the reported phonon and magnetic excitations in the system.
The power dependence is non-linear with differing exponents for the three
contributions. These results imply the presence of insulating antiferromagnetic
domains in the superconducting state of YBCO over sub-picosecond time scale,
and thereby favor an electronic phase separation picture.
