We have studied the anisotropic second-harmonic generation from the centrosymmetric semiconductors Si and Ge with the use of nanosecond and picosecond laser pulses at 1.06 and 0.53 μm. We compare our results with those of similar picosecond experiments performed by Guidotti and Driscoll [Solid State Commun. 46, 337 (1983)] and nanosecond experiments of Tom, Heinz, and Shen [Phys. Rev. Lett. 51, 1983 (1983)]. These two groups have reported seemingly different data and have offered totally different explanations for the origin and anisotropy of the effect. The picosecond results were explained by a bulk electric-dipole mechanism permitted through inversion-symmetry breaking by high-density, photoinduced carriers; the nanosecond results were claimed to originate from surface electric dipole and bulk quadrupole effects in the quiescent crystals. Our data reconcile the two earlier sets of results and are shown to be consistent with the surface-dipolar-bulk-quadrupolar model, if dispersion of the nonlinear optical parameters is taken into account.

• Received 26 November 1984

DOI:https://doi.org/10.1103/PhysRevB.31.5543