Femtosecond pump-probe near-field optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with similar to 150 nm lateral resolution, similar to 250 fs temporal resolution, and high sensitivity. This is achieved by combi ning ultrafast pump-probe laser spectroscopic techniques, which measure car rier dynamics with femtosecond-scale temporal resolution, with the nanomete r-scale lateral resolution of near-field scanning optical microscopes (NSOM s). 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 ene rgies, permitting greater measurement flexibility over earlier published wo rk. The capabilities of this instrument are proven through near-field degen erate pump-probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused-ion-beam (FIB) implantation. We f ind that lateral carrier diffusion across the nanometer-scale FIB pattern p lays a significant role in the decay time of the excited carriers within si milar to 1 mu m of the implanted stripes, an effect which could not have be en resolved with a far-field system. (C) 1999 American Institute of Physics . [S0034-6748(99)03106-8].