Ultrafast pump-probe spectroscopy with 355-nm lateral resolution at T=8 K

Summary form only given. The spatio-temporal dynamics of carriers on a submicron length scale and a subpicosecond time scale is of great interest in spectroscopy. The search for deviations from diffusive carrier transport behaviour and coherent lateral excitation transfer triggered the use of scanning near-field optical microscopy (SNOM) techniques for such investigations. However, severe technical difficulties, e.g., due to the low light throughput of optical fibers, make nonlinear experiments with a few hundred nanometers spatial resolution including broad spectral information very challenging. Our method presents an alternative that overcomes most difficulties associated with SNOM and yet allows a spatial resolution of 0.41 /spl lambda/ at helium temperature and extremely high photon flux rates. We employ a pump-probe technique that uses a combination of a long-working distance high-NA (0.4) microscope objective and a hemispherical GaP solid immersion lens with a refractive index of 3.16. The spot diameter is reduced by the refractive index n according to d/sub FWHM/=0.51/spl lambda//(n*NA). The pump and probe spots could be displaced with respect to each other in 10 nm steps. The pulse duration at the sample was 130 fs and the whole spectral information could be recorded with a standard pump-probe setup employing a spectrometer and an optical spectrum analyzer.