Quick as a Flash: Observing Ultrafast Laser-Induced Dynamics in Semiconductors

For over two decades the subject of laser-induced phase transitions in semiconductors has generated considerable interest. This is in part because of the technological interest of semiconductors in general and the annealing of ion-implanted semiconductors in particular. Recent experiments with ultrashort laser pulses suggest that an ultrafast transformation from an ordered semiconductor to a disordered metal takes place after excitation with intense femtosecond laser pulses. Using both linear and nonlinear optical techniques we have been able to directly observe the dielectric constant of GaAs during this phase transition, providing physical insight into the nature of the transition. Our data show that the laser pulse leads to a weakening of the covalent bonds in the sample. For high enough laser pulse energy, the covalent bonding vanishes and the resulting material becomes metallic.