Using the technique of tight-binding electron-ion dynamics, we have calculated the evolution of the non-linear susceptibility χ(2)(ω) in GaAs during the first few hundred femtoseconds following an ultrafast and ultra-intense laser pulse. Above a threshold fluence, our simulations show that χ(2)(ω) drops to zero, in agreement with the experimental measurements. The results indicate a rapid non-thermal transition from the original tetrahedral structure to a disordered structure, and support the conclusion that structural changes following ultrashort pulses are a direct consequence of bond destabilization.
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This work was supported by the Robert A. Welch Foundation.