This dissertation reports the response of crystalline GaAs, amorphous GaAs and thin films of amorphous GeSb when a femtosecond laser pulse excites 1--20% of the valence electrons. We developed a broadband pump-probe technique to measure the dielectric function from the near-infrared to the near-ultraviolet with a time resolution of about 100 femtoseconds. The dielectric function provides more information than ever before on the ultrafast electronic and structural dynamics and the phase changes that occur.; The dynamics depend on the excitation strength. In crystalline and amorphous GaAs, electronic effects dominate during the first few picoseconds for weaker excitations. The excited carriers affect optical properties not only through free carrier absorption, as previous experiments suggested, but also through modifications to the band structure (or allowed energy states) and filling of conduction states. Excited carriers recombine through an Auger process in crystalline GaAs and, in both phases, transfer their energy to the lattice via phonon emission. The materials consequently heat, and the dielectric function tracks the rise in lattice temperature.; For strong excitations, the dielectric function data contradict the suggestion, from reflectivity measurements at 620 nm, that GeSb films undergo a remarkable amorphous-to-crystalline transition in about 200 femtoseconds. The dielectric function we observe at this time does not match that of the thermodynamic crystalline phase. Instead the transition leads to a metal-like state that is likely to be disordered.; We observe a similar ultrafast semiconductor-to-disordered-metal transition in all three materials when the excitation is sufficiently strong. The transition can take as little as 150 femtoseconds, but it always takes longer than the pulse duration. Thus the excited electrons do not cause the change directly; rather bonds are broken when electrons are excited, the ions move to new positions and a non-thermal structural transition takes place. In all three materials, the plasma frequency of the resulting metallic state falls over time, due either to diffusion of carriers into the material or ablation from the surface.
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