This thesis presents studies of semiconductors under intense
femtosecond laser irradiation. In order to investigate the nature of
the
electronic and structural changes induced by laser pulses, a novel
broadband
technique is developed to measure the linear optical property of
semiconductors — dielectric function — over the entire visible
spectrum
(1.5–3.5 eV) with femtosecond time resolution.
By employing this broadband spectroscopic technique, the
response of
the dielectric function of GaAs following an intense 70-fs, 1.9 eV
pump pulse
is measured. The results provide the most detailed information
thus far on
the electron and lattice dynamics both above and below the
fluence threshold
for permanent damage. It is shown that electronic effects,
manifested in
changes in the band structure, dominate during the first few
hundred fs
following the excitation. After a few picoseconds, three distinct
structural
changes are observed depending upon the excitation strength: At
low pump
fluences, the dielectric function shows heating of the lattice caused
by carrier
relaxation. At intermediate fluences, the dielectric function reveals
a
temporary disordering of the lattice. At even higher fluences, a
semiconductor-to-metal transition occurs even below the damage
threshold.
The latter two effects are attributed to the lattice instability caused
by the
destabilization of the covalent bonds.
The time-integrated photoluminescence is also measured to
investigate
the dynamics of GaAs following fs laser excitation. The
luminescence images
reveal a reduction of emission due to the structural changes in
GaAs. The
spectral measurements provide new insight in the carrier
dynamics. In
addition, a series of II-VI semiconductors are also studied using
similar
techniques.
The response of crystalline Si following fs laser excitation is
also
explored using the broadband spectroscopic technique. The
dielectric
function measurements show that lattice heating and
semiconductor-to-metal transitions take place within a few
picoseconds. The long time (up to
400 ps) behavior is investigated with both reflectivity and dielectric
function
measurements, providing detailed information on the relaxation of
both
electronic and structural changes following the excitation.
Research area description 
