We report measurements of high-order harmonic spectra obtained with a 800-nm 150-fs loser 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. [S1050-2947(98)05511-5].

Time-of-flight mass spectrometry is used to study the interaction of free C60 molecules with ultrashort high power Ti:sapphire laser pulses. Ionization and fragmentation of C60 by ultrashort high-intensity laser pulses occur predominantly via plasmon excitation. Single and double excitation of collective charge oscillation is observed by remarkably large number of photons. The difference between apparent threshold energies for each ionization or fragmentation and the much lower ionization potentials is a natural consequence of primary excitation process.

A number of optical-field ionization schemes were investigated. Pulses from a high-power Ti:Sapphire laser were focused into pulsed gas targets of helium, nitrogen, oxygen and xenon. Solid target experiments were also performed.

We have investigated how high-order harmonics generated in rare gases depend on the atomic density. The peak and the profile of the atomic density in the interaction region were measured as a function of the backing pressure and the distance from the nozzle by a differential interferometry technique, The conversion efficiency for the harmonics in the plateau was found to increase approximately quadratically over the entire range of peak pressures investigated (3-80 mbar). The intensity of the harmonics in the cutoff region, in contrast, increased only until an optimum peak pressure was reached, beyond which it decreased. This optimum peak pressure was found to be dependent on both the laser intensity and the process order. To understand this effect, we have performed extensive propagation calculations of both the fundamental and the harmonic fields, using ionization rates and dipole moments from a tunnel ionization model. We obtained good agreement with the experimental results. The observed effect is attributed to ionization-induced defocusing of the fundamental laser beam, which reduces the peak intensity obtained in the medium and shortens the extent of the plateau. (C) 1996 Optical Society of America

We report a strong prepulse dependence of the emission of X-rays from plasmas created by 150 fs Ti:sapphire laser pulses on solids. The laser pulses were focused onto plane samples of aluminium and vanadium with a main pulse power density of about . By splitting off a portion of the main pulse we were able to generate a prepulse whose delay and intensity were variable relative to the main pulse. With no prepulse, very weak X-ray radiation from and was observed, whereas a prepulse to main pulse ratio of more than 0.1% and the prepulse preceding the main pulse by more than 2 ns produced up to 50 times stronger X-ray output.

We study the intensity dependence of ionization and fragmentation of buckminsterfullerene (C-60) in strong laser fields. Our data provide strong evidence that at intensities less than or similar to 10(14) W/cm(2) these processes occur predominantly via multiphoton excitation of the 20 eV plasmon resonance of C-60 At least two plasmons have to be created to initiate fragmentation or multiple ionization.

We describe the application of a tunable differential interferometer to the characterization of pulsed gas valves, operating in the low-pressure regime ( Torr). The spatial profile of the pressure in the gas jet has been studied for piezoelectric and electromagnetic valves in various experimental conditions, for both Ne and Ar gases. Moreover the time response of the valves has been investigated by using, for the first time to our knowledge, the third harmonic generation process. The number of third harmonic photons has been determined as a function of the delay time between a Nd:YAG pump laser pulse and the opening time of the valve, thus allowing the determination of the jet temporal profile. The time dependence of the local pressure in the jet has been studied for various gas pulse durations.

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The time-dependent soft X-ray emission of helium and nitrogen plasmas generated by optical-field ionization is reported. The experiments were carried out by focusing pulses of the high-power Ti:sapphire laser of the Lund Institute of Technology (λ = 796 nm, pulse duration 150 fs, pulse energy 150 mJ) to a 50-μm diameter spot close to a nozzle, using He and N 2 as target gases. The emission on He + , N 4+ , and N 3+ resonance lines was recorded by means of a flat-field grating spectrometer coupled to an X-ray streak camera. A pronounced difference in the temporal shape of the emission of the Lyman-α line of hydrogen-like helium and of the 2 p −3 d resonance lines of lithium-like and beryllium-like nitrogen was observed. The helium line exhibited an initial spike followed by a slow revival of the emission, whereas the nitrogen lines showed a slow decay after a fast initial rise. These observations are explained with the help of simulations.

High-power Ti:sapphire laser pulses (150 mJ in 150 fs) were focused into a gas jet of He, ${\mathrm{N}}_{2}$, ${\mathrm{CO}}_{2}$, ${\mathrm{O}}_{2}$, ${\mathrm{SF}}_{6}$, or Ar emitted from a pulsed nozzle. The xuv spectra that were generated are reported and analyzed. For all gases the spectra show strong lines corresponding to single-electron transitions. This indicates that the applied intensities of up to 5\ifmmode\times\else\texttimes\fi{}${10}^{16}$ W/${\mathrm{cm}}^{2}$ readily strip the atoms of all outer electrons, with three-body recombination populating excited levels of the next lower ionization stage. Simulations of the plasma evolution after the laser pulse were performed for helium and nitrogen. In the case of helium, good agreement with the experiment can be obtained for specific initial conditions. For nitrogen, mechanisms other than three-body recombination must be invoked to explain the experimental spectra. Gain measurements were made by comparing longitudinal and transverse spectra. Gains on lines connecting to the ground state or to a quasi-ground-state were not observed, but indications of gains on lines between excited states are reported.

We review the main results concerning high-order generation processes from the point of view of a potential user of this new source of XUV radiation. The perspectives for optimizing the source, both in efficiency and in spectral range, its characteristics and in particular, its coherence properties, are discussed. Finally, we describe two experiments, which demonstrate the usefulness of the harmonics as a short-pulse, coherent source in the XUV domain.

Soft X-ray spectra of He, N, C, O, S and Ar ions are reported. The emission was generated by focusing pulses of a high-power Ti:sapphire laser into a gas jet. The observed spectra are consistent with the predictions of the classical barrier suppression model. Indications of gain on lines between excited states are observed. Time-resolved observation revealed peculiar emission on He+ Lyman-alpha lines.

Soft X-ray spectra of N, O, C, S, He and Ar ions generated by optical-field ionization with fs pulses are reported. The experiments were carried out by focusing pulses of the high-power Ti:Sapphire laser of the Lund Institute of Technology (wavelength 800 nm, pulse duration 150 fs, pulse energy 150 mJ) to a 60 µm diameter spot below a pulsed gas nozzle or within a windowless pulsed gas cell, using N2, O2, CO2, SF6, He and Ar as parent gases.

High-order harmonic generation of an intense, short-pulse laser, provides a way of generating high-intensity radiation in the extreme ultraviolet (XUV) and soft X-ray spectral regions. Using the Ti:Sapphire terawatt laser in Lund (150 mJ, 150 fs at 794 nm), we have observed up to the 105th harmonic (7.6 nm) generated in He. This short-wavelength radiation has unique properties of short pulse length (~100 fs), high peak power and high spectral brightness. By focusing the harmonic radiation with suitable X-ray optics, it should be possible to obtain very high peak intensities. In this respect, high-order harmonic generation complement conventional sources of tunable XUV radiation, such as synchrotrons and laser-produced plasmas. To obtain the highest possible intensity after refocusing, it is important to maintain good temporal and spatial characteristics of the radiation as well as to optimize the number of harmonic photons generated.