S. Stagira

Toward a terawatt-scale sub-10-fs laser technology

Powerful techniques for spectral broadening and ultrabroad-band dispersion control, which allow compression of high-energy femtosecond pulses to a duration of a few optical cycles, are analyzed. Spectral broadening in a gas-filled hollow fiber and compression by chirped mirrors with high-energy 20-fs input pulses are presented. Using 1-mJ seed pulses we have demonstrated the generation of 0.5-mJ 5-fs pulses at 0.8-/spl mu/m and 1-kHz repetition rate. General design criteria to scale the compression technique toward the terawatt level are presented.

Few-optical-cycle laser pulses: from high peak power to frequency tunability

Recent advances in ultrafast laser technology have led to the generation of light pulses comprising few optical cycles employing different compression techniques. In particular, two techniques have been developed, which allow addressing the issues of high peak power or frequency tunability in a wide spectral range, namely: the hollow-fiber compression technique and the optical parametric amplification. The paper analyzes the general scheme of pulse compression and reports on the most interesting results obtained using the above-mentioned techniques. The combination of spectral broadening in a gas-filled hollow fiber with ultrabroad-band dispersion control, has led to the generation of pulses with duration of /spl sim/5 fs with peak powers up to 0.11 TW. Using optical parametric amplifiers with different configurations sub-15-fs laser pulses have been generated tunable in the visible and in the near-infrared.

Elemental sensitivity in soft x-ray imaging with a laser-plasma source and a color center detector

Elemental sensitivity in soft x-ray imaging of thin foils with known thickness is observed using an ultrafast laser-plasma source and a LiF crystal as detector. Measurements are well reproduced by a simple theoretical model. This technique can be exploited for high spatial resolution, wide field of view imaging in the soft x-ray region, and it is suitable for the characterization of thin objects with thicknesses ranging from hundreds down to tens of nanometers.

Generation of high-energy self-phase-stabilized pulses by difference-frequency generation followed by optical parametric amplification

We produce ultrabroadband self-phase-stabilized near-IR pulses by a novel approach where a seed pulse, obtained by difference-frequency generation of a hollow-fiber broadened supercontinuum, is amplified by a two-stage optical parametric amplifier. Energies up to 20 microJ with a pulse spectrum extending from 1.2 to 1.6 microm are demonstrated, and a route for substantial energy scaling is indicated.

Temporal characterization of a time-compensated monochromator for high-efficiency selection of extreme-ultraviolet pulses generated by high-order harmonics

Ultrafast extreme-ultraviolet pulses are spectrally selected by a time-delay-compensated grating monochromator. The intrinsic very short duration of the pulses is obtained by exploiting the high-order harmonic generation process. The temporal characterization of the harmonic pulses is obtained using a cross-correlation method: pulses as short as 8 fs are measured at the output of the monochromator in the case of the 23rd harmonic. This value is in agreement with the expected duration of such pulses, indicating that the influence of the monochromator is negligible. The photon flux has been measured with a calibrated photodiode, pointing out the good efficiency of the monochromator, derived by the exploitation for the two gratings of the conical diffraction mounting.

Phase-Contrast Imaging of Nanostructures by Soft X Rays from a Femtosecond-Laser Plasma

The possibility of phase-contrast imaging of nanostructures has been analyzed with the use of a femtosecond-laser plasma as a spatially coherent soft x-ray source and a LiF crystal as an x-ray detector having both the submicron spatial resolution in a wide field of view and a high contrast. It is demonstrated that the spatial coherence length of radiation in the wavelength range 1–13 nm at a distance of 30 cm from the femtosecond-laser plasma source is ≃1.5 μm. The achieved spatial coherence of the source is sufficient to obtain high-quality phase-contrast x-ray images of foils with various chemical compositions and a thickness of ≃100 nm.

Generation of fast ions in femto-and picosecond laser plasmas at low intensities of the heating radiation

X-ray spectra from Teflon targets irradiated by laser pulses with a duration of 60 fs to 1 ps have been investigated experimentally. It is shown that, when the contrast of the laser pulse is sufficiently low, the effect of self-focusing of the main laser pulse in the plasma produced by the prepulse can significantly enhance the generation efficiency of fast particles. In this case, ions with energies as high as ∼1 MeV are observed at relatively low laser intensities, qlas ≈ (4–6) × 1016 W/cm2.

Ultrafast spectroscopy of dark states in solid state sexithiophene

Femtosecond time resolved photoinduced transmission studies are carried out in nanocrystalline dihexylsexithiophene films with long range structural order. The results are compared with those obtained for sexithiophene in several states of aggregation. We explain the lack of radiative recombination in the solid phase with the formation of two nonradiative excitations, dark excitons and charge transfer states. The dark exciton dynamics is studied in detail in dihexylsexithiophene films; thermalization occurs within 200 fs, then decay takes place by mutual annihilation in the first ps, and by monomolecular recombination at longer times. Optical dynamics indicates that within the pump pulse a second excitation is formed, and we assign it to charge transfer states. The latter decay monomolecularly in the hundred ps time scale.

Surface damage of extreme-ultraviolet gratings exposed to high-energy 20-fs laser pulses

The damage fluences of gratings for diffraction of ultraviolet radiation, which are used in high-order harmonic generation experiments, have been measured with respect to the fundamental laser beam radiation. We have tested gold and platinum coatings of 40- and 50-nm thickness, respectively, deposited onto fused-silica substrates, after irradiation of high-energy, spatially filtered, 20-fs laser pulses at 780 nm. The damage appears at a fluence of approximately 0.3 J cm(-2) for gold and at a fluence of approximately 0.4 J cm(-2) for platinum. Scanning electron microscopy of the irradiated regions revealed different damaging mechanisms for the two coatings.

Photoexcitation of conjugated systems studied with sub-10 fs time resolution

The potentiality of using sub-10 fs pulses for investigating molecular spectroscopy are discussed in relation to the fundamental issue still open in the field of optical properties and dynamics of linear conjugated chains.