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We have developed a femtosecond-resolved near-field scanning optical microscope, using a diffraction-limited pump and near-field probe configuration, which allows us to measure carrier dynamics with a spatial resolution of ∼150 nm and a time resolution of ∼250 fs. This instrument is used for near-field degenerate pump–probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused-ion-beam (FIB) implantation. We find that lateral carrier diffusion across the nanometer-scale FIB pattern plays a significant role in the decay of the excited carriers.
We have quantitatively measured the linear and the nonsaturable absorption as well as the absorption modulation and its recovery time in as-grown and annealed low-temperature (LT) GaAs. Correlation of the optical data with As antisite (AsGa) defect densities yields the absorption cross section and the saturation parameter of the dominant AsGa to the conduction-band defect transition. We show that this defect transition is mainly responsible for the large nonsaturable absorption in as-grown LT GaAs with fast recovery times. Reducing the AsGa density by annealing yields an optimized material with small nonsaturable absorption, high absorption modulation, and fast recovery times.
A near-field pump-probe system with nanometer-scale spatial and femtosecond temporal resolution allows us to measure complex spatiotemporal carrier diffusion dynamics in semiconductor nanostructures. Single GaAs/Al x Ga 1Ϫx As quantum wells are patterned by nanometer-scale focused ion-beam ͑FIB͒ implantation, which introduces local carrier trapping. The resulting carrier density gradients cause diffusion, which is directly observed by measuring carrier density variations in both time and space. A comprehensive experimental study allows us to identify different diffusion regimes. We find an initial diffusion regime, characterized by nonsinusoidal carrier profiles and spatially dependent temporal diffusion decay. In a long-time regime, the carrier profile is quasisinusoidal and only weakly position-dependent temporal diffusion decay is observed.
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