This thesis describes three experiments on nonlinear optical interactions in
materials using high-energy, femtosecond laser pulses as well as several
applications of these experiments. The first one make use of linear and
nonlinear optical techniques to study ultrafast laser-induced disordering in
gallium arsenide. The pump-probe experiment is performed on both the
(100) and (110) GaAs crystalline surfaces with 165-fs, 620-nm optical pulses.
The second-harmonic generation monitors the electronic disordering induced
by the high-energy pump pulses. The linear reflectivity, on the other hand,
gives information on the variation of the dielectric constant in the highly-
excited region.
Experimental results show that the second-harmonic signal vanishes with a
decay time of 90 fs, indicating that a centrosymmetric structure is established
within the pulse width. The linear reflectivity rises to a metallic value with a
rise time of about 200 fs. Theoretical study shows that nonlinear optical
absorption processes are important for laser pulses shorter than the
electron-phonon interaction time. When a critical free carrier density is
excited, the average bonding force is weakened and cannot maintain the
crystal structure. The dielectric constant extracted from the high reflectivity
value indicates a less conducting liquid phase than equilibrium liquid GaAs.
Second-harmonic generation is also used to study Auger recombination at
high carrier density for pump pulse fluence below the disordering threshold.
The Auger recombination time is measured to be 400 fs, which requires a
screening model for explanation. The long-time lattice heating by the pump
pulse can be investigated by both linear and nonlinear optical techniques,
which give the same lattice heating time of 60 ps in GaAs.
In the second experiment, self-phase modulation in a single-mode fiber is
used to provide a synchronized 200-nm supercontinuum source generated
from the 165-fs laser pulses. Because of the Gaussian spatial profile from
the single-mode fiber and the profile-preserving amplifier cells, high-quality,
high-energy laser pulses are produced by a 10-Hz dye amplifier. A grating
pair then compresses the pulse width to 45 fs. This design provides
synchronized, tunable, femtosecond laser pulses with a Gaussian profile,
suitable for high-energy two-color pump-probe experiment and ultrafast
nonlinear laser spectroscopy.
The third experiment demonstrates the effects of self-phase modulation and
self-focusing on stimulated Raman scattering in high-pressure hydrogen
using subpicosecond laser pulses. Theoretical study of transient stimulated
Raman scattering is performed to take into account pump depletion due to
self-phase modulation. The experimental results show that the behavior of
the Raman gain falls into three input energy regimes. The Stokes radiation
production in the low input energy region can be predicted by the theory of
transient stimulated Raman scattering without pump depletion. In the
medium input energy region, strong self-phase modulation suppresses the
Stokes radiation output. This behavior is further confirmed by the addition of
argon gas. Strong self-focusing, on the other hand, breaks up the laser
beam and suppress self-phase modulation in the high input energy region.
The Stokes radiation, therefore, recovers partially.
Research area description 
