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Time-resolved studies of laser-induced phase transitions in GaAs
Yakir Siegal
Ph.D. Thesis, Harvard University, 1994, 149 pages export citation
This thesis describes a series of time-resolved experiments of
the linear
and nonlinear optical properties of GaAs during laser-induced
phase
transitions. The first set of experiments consists of a direct
determination of
the behavior of the linear dielectric constant at photon energies of
2.2 eV and
4.4 eV following excitation of the sample with 1.9-eV, 70-fs laser
pulses
spanning a fluence range from 0 to 2.5 kJ/m2. The
results from this set of
experiments were used to extract the behavior of the second-order
optical
susceptibility from second-harmonic generation measurements
made under
identical excitation conditions. These experiments are unique
because they
provide explicit information on the behavior of intrinsic material
properties -
the linear and nonlinear optical susceptibilities - during laser-
induced phase
transitions in semiconductors without the ambiguities in
interpretation that
are generally inherent in reflectivity and second-harmonic
generation
measurements.
The dielectric constant data indicate a drop in the average
bonding-
antibonding splitting of GaAs following the laser pulse excitation.
This
behavior leads to a collapse of the band-gap on a picosecond time
scale for
excitation at fluences near the damage threshold of 1.0 kJ/m
2 and even
faster at higher excitation fluences. The changes in the electronic
band
structure result from a combination of electronic screening by the
excited
free carriers and structural deformation of the lattice caused by the
destabilization of the covalent bonds. The behavior of the second-
order
susceptibility shows that the material loses long-range order
before the
average bonding-antibonding splitting, which is more sensitive to
short-
range structure, changes significantly. Loss of long-range order
and a drop of
more than 2 eV in the average bonding-antibonding splitting are
seen even at
fluences below the damage threshold, a regime in which the laser-
induced
changes are reversible.
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