DIELECTRIC-CONSTANT OF GAAS DURING A SUBPICOSECOND LASER-INDUCED PHASE-TRANSITION

We measured the time evolution of the real and imaginary parts of the dielectric constant of GaAs following femtosecond laser pulse excitati on. The data show a collapse of the average optical gap, or average bo nding-antibonding energy-level separation. The rate of collapse increa ses with pump fluence. The decrease in the gap indicates that the pump beam induces a structural transformation from a covalent, tetrahedral ly coordinated crystal to a phase with metallic cohesive properties.