▎ 摘　　要

Optical second harmonic generation (SHG) of 800 nm, 150 fs fundamental pulses is observed from exfoliated graphene and multilayer graphitic films mounted on an oxidized silicon (001) substrate. The SHG anisotropy is observed as a sample is rotated about the surface normal. For p-polarized fundamental and SHG light, the isotropic SHG from a graphene layer only slightly interferes with the fourfold symmetric response of the underlying substrate, while other samples show a threefold symmetry characteristic of significant SHG in the multilayer graphitic films. The dominance of the threefold anisotropy is maintained from bilayer graphene to bulk graphite.