Femtosecond near-field scanning optical microscopy

We have developed an instrument for optically measuring carrier dynamics in thin-film materials with approximate to 150 nm lateral resolution, approxi mate to 250 fs temporal resolution and high sensitivity. This is accomplish ed by combining an ultrafast pump-probe laser spectroscopic technique with a near-field scanning optical microscope. A diffraction-limited pump and ne ar-field probe configuration is used, with a novel detection system that al lows for either two-colour or degenerate pump and probe photon energies, pe rmitting greater measurement flexibility than that reported in earlier publ ished work. The capabilities of this instrument are proven through near-fie ld degenerate pump-probe studies of carrier dynamics in GaAs/AlGaAs single quantum well samples locally patterned by focused ion beam (FIB) implantati on We find that lateral carrier diffusion across the nanometre-scale FIB pa ttern plays a significant role in the decay of the excited carriers within approximate to 1 mu m of the implanted stripes, an effect which could not h ave been resolved with a far-field system.