Abstract

The spatio-temporal dynamics of carriers on a submicron lengthscale and a subpicosecond timescale 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 λ 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_FWHM=0.51λ/(n^*NA) [1]. 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. Figure 1(a) shows the schematic setup for the laterally resolved pump-probe experiment, and figure 1(b) shows the positions of the microscope objective, the cryostat, the solid immersion lens and the sample relative to each other.

© 2001 Optical Society of America