Utilizing a femtosecond table-top laser system, we have succeeded in converting via electron acceleration in a plasma channel, low-energy photons into antiparticles, namely positrons. The average intensity of this source of positrons is estimated to be equivalent to 2ϫ10 8 Bq and it exhibits a very favorable scaling for higher laser intensities. The advent of positron production utilizing femtosecond laser pulses may be the forerunner to a table-top positron source appropriate for applications in material science, and fundamental physics research like positronium spectroscopy.

In experiments, the feasibility was demonstrated of generating multi-MeV electrons in a form of a collimated beam utilizing a table-top laser system delivering 200 fs pulses with P L ϭ1.2 TW and 10 Hz capability. The method uses the process of relativistic self-channeling in a high-density gas jet producing electron densities in the range of 3ϫ1019 -6ϫ10 20 cm Ϫ3 . In a thorough investigation, angularly resolved and absolutely calibrated electron spectra were measured and their dependence on the plasma density, laser intensity, and gas medium was studied. For the optimum electron density of n e ϭ2ϫ10 20 cm Ϫ3 the effective temperature of the electron energy distribution and the channel length exhibit a maximum of 5 MeV and 400 m respectively. The laser-energyto-MeV-electron efficiency is estimated to be 5%. In a second step, utilizing the multi-MeV electron beam anti-particles, namely positrons, were successfully generated in a 2 mm Pb converter. The average intensity of this new source of positrons is estimated to be equivalent to a radioactivity of 2ϫ10 8 Bq and it exhibits a very favorable scaling for higher laser intensities.

Systematic studies of the temporal coherence properties of high-order harmonic radiation are presented. These complement our previous investigations ͓Bellini et al., Phys. Rev. Lett. 81, 297 ͑1998͔͒, where we showed the separation of the far-field pattern of high-order harmonics into two distinct spatial regions with different coherence times. Here we show how the coherence time of the inner and outer regions changes as a function of the harmonic order, the laser intensity, and the focusing conditions. Good agreement with the predictions of the semiclassical model of harmonic generation is obtained.

We report measurements of high-order harmonic spectra obtained with a 800-nm 150-fs laser pulse with a time-varying degree of ellipticity. The modulation of the polarization in time is achieved by using birefringent optics and self-phase modulation in a glass plate. We can create one or two temporal gates of a few femtoseconds width, during which the polarization is linear and harmonic emission is efficient. The harmonic spectra observed experimentally demonstrate that harmonics generated with linear polarization are frequency chirped. The values measured experimentally are consistent with theoretical predictions based on the strong field approximation.

Two-and three-photon ionization of rare gases using femtosecond harmonic pulses generated in a gas medium