%0 Journal Article %J Phys. Rev. B %D 1994 %T Dielectric constant of GaAs During Subpicosecond Laser-Induced Phase Transition %A Y. Siegal %A E. N. Glezer %A Mazur, E. %X We measured the time evolution of the real and imaginary parts of the dielectric constant of GaAs following femtosecond laser pulse excitation. The data show a collapse of the average optical gap, or average bonding anti-bonding energy level separation. The rate of collapse increases with pump fluence. The decrease in the gap indicates that the pump beam induces a structural transformation from a covalent, tetrahedrally-coordinated crystal to a phase with metallic cohesive properties. %B Phys. Rev. B %V 49 %P 16403–16406 %G eng %U /files/mazur/files/rep_86.pdf