Laser-induced band-gap collapse in GaAs

We present experimentally determined values of the dielectric constant of GaAs at photon energies of 2.2 and 4.4 eV following excitation of the sample with 1.9-eV, 70-fs laser pulses spanning a fluence range from 0 to 2.5 kJ/${\mathrm{m}}^2$. The data show that the response of the dielectric constant to the excitation is dominated by changes in the electronic band structure and not by the optical susceptibility of the excited free carriers. The behavior of the dielectric constant indicates a drop in the average bonding-antibonding splitting of GaAs following the laser-pulse excitation. This drop in the average splitting leads to a collapse of the band gap on a picosecond time scale for excitation at fluences near the damage threshold of 1.0 kJ/${\mathrm{m}}^2$ and on a subpicosecond time scale at higher excitation fluences. The changes in the electronic band structure result from a combination of electronic screening of the ionic potential as well as structural deformation of the lattice caused by the destabilization of the covalent bonds.