We have developed an instrument for optically measuring carrier dynamics in thin-film materials with ϳ150 nm lateral resolution, ϳ250 fs temporal resolution, and high sensitivity. This is achieved by combining ultrafast pump-probe laser spectroscopic techniques, which measure carrier dynamics with femtosecond-scale temporal resolution, with the nanometer-scale lateral resolution of near-field scanning optical microscopes ͑NSOMs͒. We employ a configuration in which carriers are excited by a far-field pump laser pulse and locally measured by a probe pulse sent through a NSOM tip and transmitted through the sample in the near field. A novel detection system allows for either two-color or degenerate pump and probe photon energies, permitting greater measurement flexibility over earlier published work. The capabilities of this instrument are proven through near-field degenerate pump-probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused-ion-beam ͑FIB͒ implantation. We find that lateral carrier diffusion across the nanometer-scale FIB pattern plays a significant role in the decay time of the excited carriers within ϳ1 m of the implanted stripes, an effect which could not have been resolved with a far-field system.

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.

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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