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The generation of harmonics by interaction of an ultrashort laser pulse with a step boundary of a plane overdense plasma layer is studied at intensities Iλ2=1017–1019 W cm−2 μm2 for normal and oblique incidence and different polarizations. Fully relativistic one-dimensional particle-in-cell (PIC) simulations are performed with high spectral resolution. Harmonic emission increases with intensity and also when lowering the plasma density. The simulations reveal strong oscillations of the critical surface driven by the normal component of the laser field and by the ponderomotive force. It is shown that the generation of harmonics can be understood as reflection from the oscillating surface, taking full account of retardation. Describing the oscillations by one or more Fourier components with adjustable amplitudes, model spectra are obtained that well reproduce the PIC spectra. The model is based on relativistic cold plasma equations for oblique incidence. General selection rules concerning polarization of odd and even harmonics depending on incident polarization are derived.

We present experimental studies on ion acceleration from ultra-thin diamond-like carbon (DLC) foils irradiated by ultra-high contrast laser pulses of energy 0.7 J focussed to peak intensities of 5 × 10 19 W/cm 2 . A reduction in electron heating is observed when the laser polarization is changed from linear to circular, leading to a pronounced peak in the fully ionized carbon spectrum at the optimum foil thickness of 5.3 nm. Two-dimensional particle-in-cell (PIC) simulations reveal, that those C 6+ ions are for the first time dominantly accelerated in a phase-stable way by the laser radiation pressure.

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Proton and ion acceleration using high-intensity lasers is a field of rapidly growing interest. For possible applications of proton beams produced in laser-solid interactions, the generation of beams with controllable parameters such as energy spectrum, brightness, and spatial profile is crucial. Hence, the physics underlying the acceleration processes has to be well understood. After the first proof-of-principle experiments [1,2], systematical studies were carried out to examine the influence of target material and thickness [3,4]. To establish the influence of the main laser parameters such as intensity, pulse energy, and duration over a wide range, results from different laser systems have to be compared, since usually each system covers a small parameter range only. Besides these parameters, strength and duration of the prepulse due to amplified spontaneous emission (ASE) play an important role, too [3]. We report on experiments carried out to establish the influence of the laser prepulse due to ASE and the target thickness on the acceleration of protons from thin aluminum foils.The protons originate from water and hydrocarbon contaminations on the foil surfaces. We used the 6-TW ATLAS laser facility at MPQ delivering 150 fs pulses at 790 nm wave length containing up to 900 mJ of energy. The pulses are focused by an f /2.5 off-axis parabolic mirror onto aluminum foils of 0.8 . . .86 µm thickness to intensities in excess of 10 19 W/cm 2 .The duration of the ASE prepulse having a peak intensity of 8 × 10 11 W/cm 2 can be controled by means of an ultra-fast Pockels cell in the laser chain. The shortest prepulse duration is 500 ps and it can be extended to several ns. The protons accelerated from the foils are detected by a Thomson parabola positioned in normal direction of the target rear side. CR 39 plates are used as a detector. The proton pits made visible by etching the CR 39 in NaOH after the shot are counted by an optical microscope equipped with a pattern-recognition software.

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