Electrostatic coupling between particles is important in many microscopic phenomena found in nature. The interaction between two isolated point charges is described by the bare Coulomb potential, but in many-body systems this interaction is modified as a result of the collective response of the screening cloud surrounding each charge carrier. One such system involves ultrafast interactions between quasi-free electrons in semiconductors-which are central to high-speed and future quantum electronic devices. The femtosecond kinetics of nonequilibrium Coulomb systems has been calculated using static and dynamical screening models that assume the instantaneous formation of interparticle correlations. However, some quantum kinetic theories suggest that a regime of unscreened bare Coulomb collisions might exist on ultrashort timescales. Here we monitor directly the temporal evolution of the charge-charge interactions after ultrafast excitation of an electron-hole plasma in GaAs. We show that the onset of collective behaviour such as Coulomb screening and plasmon scattering exhibits a distinct time delay of the order of the inverse plasma frequency, that is, several 10(-14) seconds.

Bandwidth-limited infrared pulses as short as 50 fs are generated in thin GaSe crystals by phase-matched optical rectification of 10 fs laser pulses. The central frequency of the transients is continuously tunable over a wide interval extending from 41 THz (λ=7 μm) to the far-infrared. The electric field of the THz transients is directly monitored via ultrabroadband free-space electro-optic sampling. A simulation of the spectra based on a plane-wave model shows excellent agreement with the experiment.

Abstract. Live microscopy techniques ͑i.e., differential interference contrast, confocal microscopy, etc.͒ have enabled the understanding of the mechanisms involved in cells and tissue formation. In long-term studies, special care must be taken in order to avoid sample damage, restricting the applicability of the different microscopy techniques. We demonstrate the potential of using third-harmonic generation ͑THG͒ microscopy for morphogenesis/embryogenesis studies in living Caenorhabditis elegans ͑C. elegans͒. Moreover, we show that the THG signal is obtained in all the embryo development stages, showing different tissue/structure information. For this research, we employ a 1550-nm femtosecond fiber laser and demonstrate that the expected water absorption at this wavelength does not severely compromise sample viability. Additionally, this has the important advantage that the THG signal is emitted at visible wavelengths ͑516 nm͒. Therefore, standard collection optics and detectors operating near maximum efficiency enable an optimal signal reconstruction. All this, to the best of our knowledge, demonstrates for the first time the noninvasiveness and strong potential of this particular wavelength to be used for highresolution four-dimensional imaging of embryogenesis using unstained C. elegans in vivo samples. © 2010 Society of Photo-Optical Instrumentation Engineers.

In a non-equilibrium electron-hole plasma photoexcited in GaAs within 10 fs, we observe how a plasmon pole builds up on a femtosecond time scale. To this end, the dynamics of the complex dielectric function throughout the mid-infrared is probed with uncertainty-limited temporal resolution by means of ultrabroadband THz spectroscopy. We show that collective effects, such as plasma oscillations, exhibit a delayed onset. This observation may be linked to the ultrafast formation of dressed quasiparticles in the many-body system. The approximate time scale for these phenomena is found to be of the order of the inverse plasma frequency. Thus, our findings support quantum kinetic models of the temporal evolution of the Coulomb interaction after ultrafast carrier generation in semiconductors. These theories have been co-pioneered by R. Zimmermann and his coworkers around the beginning of the last decade.

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