Annual Meeting of the American Physical Society, March 1998, Los Angeles, CA

Session C15 - Phonons I.
MIXED session, Monday morning, March 16
406A, Los Angeles Convention Center

[C15.08] LO Phonon Hole-Plasma Coupling Near the Surface of GaAs (110) After Femtosecond Lase r Irradiation

We have studied coherently excited longitudinal optical phonons near the GaAs surface in the presence of a dense hole plasma created after femtosecond laser pulse irradiation. A 20 femtosecond, 840 nm, pump laser pulse injects electron-hole pairs near the surface of our n-doped GaAs (110) sample. We detect the second-harmonic generated by a second time-delayed probe pulse. The depletion field deforms the lattice by the piezoelectric effect. When the injected carriers screen the field, the lattice is left to freely oscillate in a long-wavelength LO phonon mode. [1]The injected carriers screen the depletion field in ~10 fs, creates electron-hole pairs in the depletion field near the surface of our n-doped GaAs (110) sample. The depletion field deforms the lattice by the piezoelectric effect--when the field is rapidly screened by carriers, the lattice is left to freely oscillate in a long-wavelength LO phonon mode.[1] These coherent LO phonons are observed as periodic modulations in time-resolved second-harmonic generation from a second time-delayed probe pulse.[2] We will discuss the injected carrier density on both the frequency and dephasing time of the phonon-hole-plasma coupled mode. [1] G.C. Cho, W. Kutt and H. Kurz, Phys. Rev. Lett., 65. 764 (1990). [2] Y.-M. Chang, L. Xu, and H.W.K. Tom, Phys. Rev. Lett. 78, 4649 (1997).