▎ 摘　　要

We have experimentally investigated the optical second harmonic generation (SHG) on suspended single-layer and bi-layer graphene sheets. By shining normally incident 800-nm light with polarization along the sample planes, the SHG intensities of single-layer and bi-layer graphene are found to be comparable to the one of polar GaAs with large second order susceptibility, which is unexpected because both have the centrosymmetric property. Our experimental results reveal that the strong SHG is not due to the defects breaking the symmetry. Instead, we suggest that the SHG signals result from the long-range curvature fluctuations on the suspended single-layer and bi-layer graphene sheets. (C) 2014 AIP Publishing LLC.