Nonequilibrium materials: using ultrafast laser pulses to change band structures

Soon after it was discovered that intense laser pulses of nanosecond duration from a ruby laser could anneal the lattice of silicon, it was established that this so-called pulsed laser annealing is a thermal process. The past two decades have show that ultrashort laser pulses in the femtosecond regime can induce athermal, nonequilibrium processes that lead to either transient phase changes in semiconductors through ultrafast ionization or permanent phase changes through nonequilibrium doping. In this talk we will review work in both of these regimes and show how ultrafast lasers can be used to collapse the bandgap of a semiconductor or to hyperdope a semiconductor far above the solubility limit, leading to the formation of an intermediate band. This work not only reveals important information on the electron dynamics but also permits creating materials with novel properties.