Session C15 - Phonons I.
MIXED session, Monday morning, March 16
406A, Los Angeles Convention Center
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. 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. These coherent LO phonons are observed
as periodic modulations in time-resolved second-harmonic generation from
a second time-delayed probe pulse. We will discuss the injected carrier
density on both the frequency and dephasing time of the phonon-hole-plasma
coupled mode.  G.C. Cho, W. Kutt and H. Kurz, Phys. Rev. Lett., 65.
764 (1990).  Y.-M. Chang, L. Xu, and H.W.K. Tom, Phys. Rev. Lett. 78,