Phase Transitions Induced By Femtosecond Pulses
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ABSTRACT Optical studies of semiconductors under intense femrtosecond laser pulse excitation suggest that an ultrafast phase transition takes places before the electronic system has time to thermally equilibrate with the lattice. The excitation of a critical density of valence band electrons destabilizes the covalent bonding in the crystal, resulting in a structural phase transition. The deformation of the lattice leads to a decrease in the average bonding-antibonding splitting and a collapse of the band-gap. Direct optical measurements of the dielectric constant and second-order nonlinear susceptibility are used to determine the time evolution of the phase transition. INTRODUCTION Intense, femtosecond laser-pulse excitation of semiconductors provides a unique opportunity for observing the dynamics of a phase transition. The semiconductor-metal transition that can result from such excitation- 11 is particularly interesting because it illustrates the critical role high free-carrier densities can play in modifying the electronic and structural properties of semiconductors. Understanding the complex dynamics involved in laser-induced phase transitions requires an explicit determination of the behavior of intrinsic material properties during these transitions. Dielectric Constant Without direct determination of the time-evolution of the dielectric constant, interpretation of reflectivity and second-harmonic data has relied to date on making assumptions about the functional form of the dielectric constant. Specifically, it has been assumed that the changes in the dielectric constant induced by the excitation are dominated by the free carrier contribution to the optical susceptibility. Under this assumption, the changes in the dielectric constant have been modeled using a Drudemodel formalism.1, 7,12 While this type of assumption is legitimate at lower excitation regimes, it is misleading in the case of laser-induced disordering experiments. Misinterpretation of the data arises because a single-incident-angle reflectivity value does not correspond to a unique value of the dielectric constant. Thus, one can reproduce single-incident-angle reflectivity data using dielectric constant values that are completely different from the actual ones. To avoid relying on an assumed functional form for the dielectric constant in interpreting the results of femtosecond pump-probe experiments on GaAs, we directly determined the time evolution of the real and imaginary parts of the dielectric constant. Specifically, we experimentally determined the behavior of the complex dielectric constant at photon energies of 2.2 eV and 4.4 eV following excitation with an intense, 70fs pump pulse at 1.9 eV. To uniquely extract the real and imaginary parts of the dielectric constant, two independent measured quantities are necessary. Therefore, at each probe frequency we measured the p-polarized reflectivity at two carefully chosen angles of Mat. Res. Soc. Symp. Proc. Vol. 397 01996 Materials Research Society
incidence using two simultaneous 70-fs p
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