title: Infrared-phonon-polariton resonance of the nonlinear susceptibility in GaAs.
abstract: Nonlinear probing of the fundamental lattice vibration of polar crystals is shown to reveal insight into higher-order cohesive lattice forces. With a free-electron laser tunable in the far infrared we experimentally investigate the dispersion of the second-order susceptibility due to the phonon resonance in GaAs. We observe a strong resonance enhancement of second harmonic light generation at half the optical phonon frequency, and in addition a minimum at a higher frequency below the phonon frequency. Measuring this frequency and comparison to a theoretical model allows the determination of competing higher-order lattice forces.
authors.affiliation: Institute of Ion Beam Physics and Materials Research, Forschungszentrum Rossendorf, P.O. Box 510119, 01314 Dresden, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Dekorsy
authors.forename: V A
authors.initials: VA
authors.lastname: Yakovlev
authors.forename: W
authors.initials: W
authors.lastname: Seidel
authors.forename: M
authors.initials: M
authors.lastname: Helm
authors.forename: F
authors.initials: F
authors.lastname: Keilmann