This thesis describes three experiments using pump-probe
spectroscopy
with picosecond and femtosecond laser pulses to study carrier
dynamics in
semiconductors. In the first experiment, highly excited gallium
arsenide is
studied with reflectivity and reflection second-harmonic probes
using 160-fs
pulses at 623 nm. Above a threshold incident fluence of ~0.1 J/
cm2, the
second-harmonic signal is observed to fall to zero in 100 fs. This
drop shows
that the valence electrons undergo a transformation to a
centrosymmetric
configuration and strongly suggests that the atomic lattice
disorders before
acquiring appreciable energy from the pulse. With an exponential
time of 200
fs the reflectivity rises to a steady high value that is consistent with
a metallic
molten phase. Tens of picoseconds after excitation, the reflectivity
drops
considerably from the high value of the liquid and probe light is
increasingly
scattered out of the plane of incidence. The excitation produces
~90-nm-deep pits in the wafer surface that are covered with a layer
of solidified
droplets. An estimate of the absorption and scattering caused by a
cloud of
liquid droplets ejected from the surface suggests that ablated
material can
account for the reflectivity drop.
In the second experiment, femtosecond transient absorption
spectroscopy is used to study the carrier dynamics of type II GaAs/
AlAs
multiple quantum wells. The spectra show a rapid partial recovery
in the
pump-induced bleaching near the absorption edge that is
produced by the
rapid scattering of conduction electrons in the Gamma valley of the
GaAs
layers to the X valleys of the AlAs barrier layers. The scattering
time is
measured to be 100 fs for an 8-monolayer sample and 400 fs for
an 11-monolayer sample.
In the third experiment, the picosecond laser melting of silicon
is studied
using a streak camera to provide spatial and temporal resolution.
The
reflectivity near Brewster’s angle shows the expected eightfold
increase
following melting. Images of the excited silicon surface emphasize
the
importance of spatial resolution in nearthreshold experiments.
Research area description 
