  We investigate the effect of nitrogen-vacancy (NV) centers in single crystal
diamond on nonlinear optical effects using 40 fs femtosecond laser pulses. The
near infrared femtosecond pulses allow us to study purely nonlinear optical
effects, such as optical Kerr effect (OKE) and two-photon absorption (TPA),
relating to unique optical transitions by electronic structures with NV
centers. It is found that both the nonlinear optical effects are enhanced by
the introduction of NV centers in the N$^{+}$ dose levels of
2.0$\times$10$^{11}$ and 1.0$\times$10$^{12}$ N$^{+}$/cm$^{2}$. In particular,
our data demonstrate that the OKE signal is strongly enhanced for the heavily
implanted type-IIa diamond. We suggest that the strong enhancement of the OKE
is possibly originated from cascading OKE, where the high-density NV centers
effectively break the inversion symmetry near the surface region of diamond.
