  We investigated the terahertz (THz)-pulse driven nonlinear response in the
d-wave cuprate superconductor Bi2Sr2CaCu2O8+x (Bi2212) using a THz pump
near-infrared probe scheme in the time domain. We have observed an oscillatory
behavior of the optical reflectivity that follows the THz electric field
squared and is strongly enhanced below Tc. The corresponding third-order
nonlinear effect exhibits both A1g and B1g symmetry components, which are
decomposed from polarization-resolved measurements. Comparison with a BCS
calculation of the nonlinear susceptibility indicates that the A1g component is
associated with the Higgs mode of the d-wave order parameter.
