  We have measured the temperature dependent third harmonic response from a
series of under-doped YBa_2Cu_3O_7-d thin films to address the mechanism of
nonlinearity in these samples. We find that the intrinsic Ginzburg-Landau
nonlinearity near Tc is doping dependent, with the sample becoming more
nonlinear as it is under-doped. The results are consistent with the doping
dependence of the condensation energy of YBa_2Cu_3O_7-d.
