GAAS UNDER INTENSE ULTRAFAST EXCITATION - RESPONSE OF THE DIELECTRIC FUNCTION

We used a new broadband spectroscopic technique to measure the dielect ric function of GaAs over the spectral range of 1.5-3.5 eV following i ntense 70-fs laser excitation. The results provide the most detailed i nformation thus far on the electron and lattice dynamics both above an d below the fluence threshold for permanent damage, F-th = 1.0 kJ/m(2) . There are three distinct regimes of behavior: lattice heating (<0.5F (th)), lattice disordering (0.6-0.8)F-th, and a semiconductor-to-metal transition (>0.8F(th)). Below F-th, the changes are completely revers ible.