Spatially resolved femtosecond spectroscopy beyond the diffraction limit

Abstract

Femtosecond spectroscopy with 250 fs temporal and 150 nm spatial resolution is made possible by combining near-field scanning optical microscopy with ultrafast pump-probe techniques. Femtosecond-resolved near-field microscopy allows transport studies in nanostructured materials as well as the study of carrier dynamics in single nanostructures with unprecedented temporal and spatial resolution. Measurements on single nanostructures are a powerful means for the characterization of the inhomogeneity of nanostructure ensembles. We describe the design and performance of femtosecond near-field scanning optical microscopes and give examples of typical femtosecond-resolved near-field experiments on semiconductor nanostructures.

Ultrahigh spatiotemporal resolved spectroscopy

“…For the widely concerned mesoscopic systems and nanomaterials, because of their small characteristic dimensions, a thorough study on their ultrafast dynamics needs ultrahigh resolutions in both space and time to be obtained simultaneously. The same conclusion is reached if one considers the research of real-space transport, since a transportation electron moves over a distance on the order of nanometer in one femtosecond [3] . Here, the small space-scale and the small time-scale are tightly related with each other.…”

“…Siegner et al [3,17,18] used GaAs single quantum wells as samples. Periodical nanometer-width implantation stripes were laterally patterned on these samples by focused-ion-beam (FIB) implantation.…”

“…The other measures the pump-probe signal versus time while fixing the SNOM tip position, thus telling us the temporal evolvement of the carrier density at a given position. The full lines in Figure 3 are results under the first data collection mode at time delays of Δt = 4 ps and Δt = 80 ps, respectively [3,18] . They display the pump-probe signals versus distance across the FIB implanted stripes.…”

“…With the increase of the time delay, the profile gets smoother, which reflects the effect of strong carrier diffusion driven by the steep gradient of the carrier density close to the implanted stripes. The full lines in Figure 4 are results under the second data collection mode, which show the temporal evolvement of the carrier densities at the center of stripe (x = 0.0 μm), 400 nm from the center (x = 0.4 μm) and 800 nm from the center (x = 0.8 μm) [3,18] . Carrier relaxation at the stripe center is the fastest due to the main contribution of fast trappings by defects and less contribution of the slow recombination process.…”

“…Siegner and Achermann et al [3,19] measured the quantization energy fluctuations along a single V-groove quantum wire (QWR), using the same experimental setup as in the previous quantum well studies. Figure 5 shows the near-field pump-probe traces taken at the same position of the QWR but at various photon energies around the lowest exciton resonance.…”