Ultrafast mid-infrared laser pulse generation using optical semiconductor switching is investigated experimentally for application to subpicosecond Co₂ laser pulse generation at 10.6 μm. Time-resolved infrared measurements, which are based on cross-correlation and reflection-reflection correlation techniques, are used to determine the duration of the reflected infrared pulses from a GaAs infrared reflection switch. These time-resolved measurements together with time-integrated measurements are used to derive a model describing the behaviour of the GaAs infrared reflection switch. it is found that diffusion and two-body recombination whose rate is taken to be density-dependent, can accurately describe the ultrafast infrared reflectivity switching process in GaAs. We have also in vestigated some novel semiconductor materials with ultrafast recombination lifetimes for ultrafast semiconductor switching application. A molecular beam epitaxy low tempera ture grown GaAs (LT-GaAs) and radiation damaged GaAs (RD-GaAs) are successfully used to switch out ultrashort Co₂ laser pulses. Application of the time-resolved crosscorrelation technique to nonequilibrium carrier lifetime measurements in highly excited LT-GaAs, RD-GaAs, and In₀.₈₅Ga₀.₁₅As/GaAs relaxed superlattice structure are found to be in good agreement with other reported techniques. As an application to semicon ductor probing, ultrafast infrared transmission experiments are conducted to determine the absorption of infrared pulses in Si of various dopings after free carriers have been generated by absorption of a subpicosecond laser pulse of above band gap photon en ergy. By fitting the experimental data to a theoretical model, the free-carrier absorption cross-sections and the momentum relaxation times are calculated.