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Optical studies of semiconductors under intense femtosecond laser pulse excitation suggest that an ultrafast phase transition takes place 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 bondingantibonding splitting and a collapse of the bandgap. We review the relationship between structural, electronic, and optical properties, as well as the timescales for electron recombination, diffusion, and energy relaxation. Direct optical measurements of the dielectric constant and secondorder nonlinear susceptibility are used to determine the time evolution of the phase transition.

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