From Semiconductor to Metal in a Flash: Observing Ultrafast Laser-Induced Phase Transformations

We use a new broadband spectroscopic technique to measure ultrafast changes in the dielectric function of a material over the spectral range 1.5-3.5 eV following intense 70-fs laser excitation. The results reveal the nature of the phase transformations which occur in the material following excitation. We studied the response of GaAs and Si. For GaAs, there are three distinct regimes of behavior as the pump fluence is increased — lattice heating, lattice disordering, and a semiconductor-to-metal transition.