Fourth-harmonic Generation at Crystalline Si(001) Interfaces

Summary form only given. We previously reported optical fourth-harmonic generation (FHG) in reflection from a GaAs(001) surface using 200-fs pulses, well below the damage threshold.' The data revealed a strong fourfold anisotropic surface-specific polarization that is not present in second-harmonic generation (SHG) and that, surprisingly, was nearly as strong as the dipole-allowed, twofold anisotropic contribution from the noncentrosymmetric bulk. The unusual relative strength of the surface FH contribution suggested that surface-specific FHG could be observed by itself in reflection from a centrosymmetric crystal. In this work, we demonstrate azimuthally anisotropic FHG from a crystalline Si(OO1) interface using 5-pJ, 200-fs pulses at fluences >5X below the damage threshold. Our data indicate that the surface dipole FH contribution dominates over the bulk quadrupole FH contribution much more strongly than for the corresponding SHG contributions, thus making FHG from Si(OO1) one of the most purely surface-specific optical probes ever reported.