On the feasibility of performing non-linear autocorrelation with attosecond pulse trains

We present experimental results in which a second-order effect, namely two-photon ionization of atomic He induced by a superposition of harmonics, is observed. The harmonics are generated in a Xe gas-jet using a 790-nm 10-Hz femtosecond Ti:sapphire laser and are subsequently focused into a He gas-jet with a Kirkpatrick–Baez arrangement. The superposition is formed by using a thin In filter and it comprises the 7th to 13th harmonics. Solving the time-dependent Schrödinger equation for He in a polychromatic laser field, the He ⁺ ion yield is calculated as a function of the total XUV intensity. Using the calculated yield and taking into account the focusing and transmission properties of the arrangement, the number of He ⁺ ions produced per laser pulse is estimated and is found to be in reasonable agreement with its measured value. The total number of ions produced non-resonantly follows a nearly quadratic dependence on the harmonic intensity, thus establishing the feasibility of a second-order auto-correlation measurement of the superposition of harmonics, i.e., of a direct temporal characterization of attosecond pulse trains.     

Toward single attosecond pulses using harmonic emission from solid-density plasmas

We report on investigations of high-order harmonic generation from solid surfaces in the coherent wake emission regime with relativistically intense few-cycle (8 fs) laser pulses. Significant spectral broadening compared to previous experiments with many-cycle pulses and the appearance of substructures on the harmonics are observed that strongly fluctuate from shot-to-shot. Measurements in which the linear polarization was rotated or ellipticity of the laser pulse was varied exhibit a strong dependence of the harmonic emission on the polarization state of the incident pulse. We show that the observed spectral features are ultimately connected to the sub-cycle electron dynamics in the laser-solid interaction and thus proof of the few-cycle nature of the observed harmonic emission. Using a simple model we have investigated the factors that play an important role in the shape of the emitted spectrum.     

Percolation of randomly distributed growing clusters: the low initial density regime

We investigate the problem of growing clusters, which is modeled by two dimensional disks and three dimensional droplets. In this model we place a number of seeds on random locations on a lattice with an initial occupation probability, p. The seeds simultaneously grow with a constant velocity to form clusters. When two or more clusters eventually touch each other they immediately stop their growth. The probability that such a system will result in a percolating cluster depends on the density of the initially distributed seeds and the dimensionality of the system. For very low values of p we find a power law behavior for several properties that we investigate, namely for the size of the largest and second largest cluster, for the probability for a spanning cluster to occur, and for the mean radius of the finally formed droplets. We report the values of the corresponding scaling exponents. Finally, we show that for very low initial concentration of seeds the final coverage takes a constant value which depends on the system dimensionality.     

Ultra-high-contrast few-cycle pulses for multipetawatt-class laser technology

We report the generation of few-cycle multiterawatt light pulses with a temporal contrast of 10(10), when measured as close as 2 ps to the pulse's peak. Tens of picoseconds before the main pulse, the contrast value is expected to spread much beyond the measurement limit. Separate measurements of contrast improvement factors at different stages of the laser system indicate that real contrast values may reach 10(19) and 10(14), when measured 50 and 25 ps before the pulse's peak, respectively. The combination of the shortest pulse duration and the highest contrast renders our system a promising front-end architecture for future multipetawatt laser facilities.     

Extending optical fs metrology to XUV attosecond pulses

We report on the progress towards a 2nd-order autocorrelation measurement of a coherent superposition of the 7th to the 15th harmonic of the fundamental frequency of the Ti : Sapphire laser. The two-photon ionization of He atoms was investigated and found to be an appropriate non-linear detector. In addition, the technique of volume autocorrelation based on a split mirror wavefront divider was analysed and shown to exhibit adequate degree of modulation. Moreover, the approach presented opens up the path to XUV-pump XUV-probe type of temporal studies of ultrafast processes with a sub-fs temporal resolution. PACS 42.60.By; 42.65.Re; 42.65.Ky     

Two-dimensional unsaturated flow in irregularly shaped regions using a finite volume method

To describe the two-dimensional flow of water in unsaturated soil, the governing equation is solved on a mesh constructed from small area elements. A transformation is introduced with which these possibly distorted rectangular elements of the physical plane are mapped into computational squares. Thus, irregularly shaped regions, which present difficulties when attempting to describe their geometry on an orthogonal computational mesh, can be more easily modelled. Using this methodology, here called the finite volume method, numerical results are obtained showing the ability of the method to describe transient unsaturated flow.     

A dispersionless Michelson interferometer for the characterization of attosecond pulses

A novel application of a free-standing transmission grating as a beam splitter in a Michelson-type interferometer is described. The arrangement can operate in the XUV and soft X-ray spectral region and, therefore, it is well suited for the characterization of attosecond pulses. Using ray-tracing codes, we have analyzed three different setups in which spherical mirrors are employed in conjunction with the transmission grating and have investigated in detail their dispersive characteristics. It is shown that such an arrangement can be made to exhibit group-delay dispersion of ∼1 as² while it provides two co-propagating and converging beams. Received: 12 October 2001 / Revised version: 17 December 2001 / Published online: 7 February 2002     

Determination of nonneutral plasma microstate from synchrotron radiation measurements

The spectrum of the synchrotron radiation emitted from the University of Maryland mirror confined nonneutral plasma is measured during the decay of the pulsed magnetic field and used to obtain the energy distribution ?(γ) and electron density profile n_e⁰(r). One result is that the electron density profile evolves from a relatively thin E-layer to a broad profile with density build-up on axis.     

Time-sequence imaging of relativistic laser–plasma interactions using a novel two-color probe pulse

We describe and demonstrate a novel experimental technique to record two picosecond-separated images of the evolution of a single-shot laser-produced plasma using a two-color probe beam. In this technique, the probe beam was first partially frequency doubled. The 1ω and 2ω components were then temporally separated in a dispersive medium. The pulses remained intrinsically synchronized, both having a duration of ∼150 fs, but they were separated by 4.1 ps. Using a Nomarskii-type interferometer, this novel technique was used to obtain two time-resolved snapshots of the evolution of the electron density at different time steps during the formation of a relativistic plasma channel.     

Influence of the laser prepulse on proton acceleration in thin-foil experiments

We investigate the influence of the laser prepulse due to amplified spontaneous emission on the acceleration of protons in thin-foil experiments. We show that changing the prepulse duration has a profound effect on the maximum proton energy. We find an optimal value for the target thickness, which strongly depends on the prepulse duration. At this optimal thickness, the rear side acceleration process leads to the highest proton energies, while this mechanism is rendered ineffective for thinner targets due to a prepulse-induced plasma formation at the rear side. In this case, the protons are primarily accelerated by the front side mechanism leading to lower cutoff energies.