Spatial characterization of high harmonics (HH) and XUV coherent radiation is of paramount importance, along
with its temporal characterization. For many applications it will be necessary to accurately measure the beam
properties, just as it is important to know the beam characteristics for many laser experiments. For example,
high harmonics and attosecond pulses are being proposed as a front-end for the next generation X-ray free
electron lasers. This oscillator-amplifier-like arrangement will require well characterized high harmonic sources.
On the other hand, the electromagnetic radiation carries the combined signature of underlying quantum physical
processes at the molecular level and of the cooperative phase matching. For example, accurate reconstruction of
the high harmonic spatial wavefront, along with its temporal profile, gives us a complete range of tools to apply
to the fundamental quantum properties and dynamics associated with high harmonic generation. We present
a new concept of frequency resolved wavefront characterization that is particularly suitable for characterizing
XUV radiation. In keeping with tradition in the area we give it an acronym - SWORD (Spectral Wavefront
Optical Reconstruction by Diffraction). Our approach is based on an analysis of the diffraction pattern of a slit
situated in front of a flat-field spectrometer. As the slit is scanned, the spectrally resolved diffraction pattern
is recorded. Analyzing the measured diffractogram, we can reconstruct the wavefront. The technique can be
easily extended beyond the XUV spectral region. When combined with temporal characterization techniques all
information about the beam can be measured.
We report on the first experiments of high-order harmonic generation done with the 100 Hz high-energy optical
parametric amplifier (OPA) of the Advanced Laser Light Source. Using krypton and argon as targets, we show that the
OPA's signal beam − with a wavelength range from 1200 nm to 1600 nm, 1.3 mJ to 0.8 mJ of pulse energy and 100 fs
pulse duration − can generate fully tunable XUV radiation down to a wavelength of 15 nm. We have also started to
investigate the use of the OPA pulses for molecular imaging. Inducing molecular alignment with 800 nm, 70 fs pulses,
we have measured the high harmonics spectra generated with 1300 nm pulses from nitrogen molecules oriented at
various angles with respect to the ionizing field, in order to study for the first time the technique of molecular orbital
tomography with a laser wavelength different than 800 nm.
High harmonics produced in aligned molecules contain the structural information of bound-state electronic states. We
have produced high harmonics from N<sub>2</sub> molecules aligned to arbitrary directions with 5-degrees steps. From the set of
high harnionic spectra, we have successfully reconstructed tomographic images of the highest occupied molecular
orbital (HOMO) of N<sub>2</sub>.
High harmonics produced in aligned molecules contain the structural information of the outermost electron orbital that preferentially ionizes in intense laser fields. We show a method to reconstruct a 3-dimensional (3D) structure of the molecular orbital. The method is based on the technologies to align molecules and to produce attosecond XUV pulses, both of which utilize intense ultrashort laser pulses. We measured a set of high harmonic spectra produced in differently aligned N<sub>2</sub> molecules, and successfully reconstructed the image of the highest occupied molecular orbital (HOMO) with sub-angstrom resolution.
We have investigated the full three dimensional momentum correlation between the electrons emitted from strong field double ionization of neon when the re-collision energy of the first electron is on the order of the ionization potential of the singly charged neon ion. We find that the momentum correlation in the direction perpendicular to the laser field depends on the time difference of the two electrons leaving the ion. Our results are consistent with double ionization proceeding through transient double excited states that field ionize.
Thomson scattering, X-ray diagnostics and optical interferometry are employed to characterize both collisionally excited and recombination X-ray laser plasmas. Spatially and time-resolved Thomson scattering spectra from carbon plasmas provide the electron and ion temperatures, which are compared with hydrodynamic computer modelling. The electron density is inferred from the recorded interferograms. Time-integrated X-ray spectra in the keV range are used to characterize very precisely the optimum irradiance conditions for lasing in Ne-like germanium plasmas. Also, electron temperatures obtained with the 5C/4C line ratio, assuming a PLTE model, are compared to Thomson scattering results.
Slab germanium targets have been irradiated in a line focus geometry with 1-3 nsec FWHM, 1.06-micron laser pulses at irradiances of I equal to or less than 10 exp 13 W/sq cm. The effect of varying the rise-time of the driving laser pulse on the amplification of 3-3 soft X-ray lasing lines is investigated. Results of short-pulse (100 psec FWHM) experiments have also shown gain on the same lasing transitions, but at substantially higher irradiances of I equal to or greater than 3 x 10 exp 13 W/sq cm.
Measurements of reflectivity of stimulated Brillouin scattering, from an underdense, homogeneous plasma, irradiated by a 10 ps-1.06μm laser pulse, show an increase from 10<sup>-4</sup> to 10<sup>-2</sup>, as the laser intensity is varied between 10<sup>13</sup> to 10<sup>15</sup> W/cm<sup>2</sup>. Numerical simulations have been developed to interpret these data. At low laser intensity, (<10<sup>14</sup> W/cm<sup>2</sup>), the low reflectivity is well explained as being due to saturation of the instability in the convective regime, and good agreement with numerical simulations has been obtained. At high laser intensity, the model predicts higher reflectivities than measured in the experiment, although non-linear effects in ion sound waves evolution have been observed.
High Z exploding foil targets are used in many soft x-ray laser schemes. The foils
are typically irradiated with a long ('1 ns) optical laser pulse that burns through the
foil to produce a large hot plasma with long density scale lengths suitable as an
amplifier. While it is expanding, and before the plasma conditions are suitable for
stimulated emission, a large fraction of the heating laser energy is lost through
radiation and conduction. We report the results of experiments attempting to
increase the efficiency of exploding foil amplifiers through the following procedure.
A laser beam ('1 2 W/cm2) is used to heat the foil sufficiently to expand it to
approximately 200 pm. A high intensity (''1 5 W/cm2) short pulse (1 0 ps) laser
beam is then used to raise the plasma to the desired temperature and ion state.
Temporally resolved x-ray spectra from Yb foils are presented.
We present in this paper observations of stimulated Briiiouin backscattered light from a plasma
irradiated by a 1 0 ps 1 .06 m laser pump, at intensities between 1 012 and 1 016 W cm2. During this short
time duration, the observed growth of the instability can be explained only in terms of a purely temporal
growth, in contrast to the convective growth normally associated with this instability.