We have measured third-order nonlinear susceptibility $\left({\chi }^{\mathrm{}\left(3\right)\mathrm{}}\right)$ spectra of the two-dimensional (2D) cuprates ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ by third-harmonic generation. The $|{\chi }^{\mathrm{}\left(3\right)\mathrm{}}|$ spectra exhibit broad peaks with maximum values of $0.6\times {10}^{-10}\mathrm{esu}$ in ${\mathrm{La}}_2{\mathrm{CuO}}_4$ and $2\times {10}^{-10}\mathrm{esu}$ in ${\mathrm{Nd}}_2{\mathrm{CuO}}_4.$ These peaks are attributable to three-photon resonance to the charge transfer (CT) excited state. The ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ phase spectrum of ${\mathrm{Nd}}_2{\mathrm{CuO}}_4$ has revealed that the two-photon resonance to the even CT state are hidden on the high-energy side of the ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ peak. The difference in ${\chi }^{\mathrm{}\left(3\right)\mathrm{}}$ values between the 2D and 1D cuprates is discussed by considering differences in spin-charge coupling and in charge configurations between the 2D and 1D cases.

• Received 10 January 2003

DOI:https://doi.org/10.1103/PhysRevB.68.075101