GaAs under Intense Ultrafast Excitation: Response of the Dielectric Function

We used a new broadband spectroscopic technique to measure the dielectric function of GaAs over the spectral range of 1.5--3.5 eV following intense 70-fs laser excitation. The results provide the most detailed information thus far on the electron and lattice dynamics both above and below the fluence threshold for permanent damage, Fₜₕ0ex0ex=0ex0ex1.00ex0exkJ/m². There are three distinct regimes of behavior: lattice heating (<0.5Fₜₕ), lattice disordering (0.6--0.8) Fₜₕ, and a semiconductor-to-metal transition (>0.8Fₜₕ). Below Fₜₕ, the changes are completely reversible.