title: Fourth-harmonic generation at a crystalline GaAs(001) surface.
abstract: We demonstrate fourth-harmonic generation from a GaAs(001) surface by using femtosecond pulses well below damage threshold. Our data reveal a strong fourfold anisotropic surface-specific polarization that is not present in second-harmonic generation and is nearly as strong as the bulk polarization.
authors.forename: Y S
authors.initials: YS
authors.lastname: Lee
authors.forename: M H
authors.initials: MH
authors.lastname: Anderson
authors.forename: M C
authors.initials: MC
authors.lastname: Downer