High-repetition rate attosecond pulse sources are indispensable tools for time-resolved studies of electron dynamics, such as coincidence spectroscopy and experiments with high demands on statistics or signal-to-noise ratio, especially in the case of solid and big molecule samples in chemistry and biology. Although with the high-repetition rate lasers, such attosecond pulses in a pump-probe configuration are possible to achieve, until now, only a few such light sources have been demonstrated. Here, by shaping the driving laser to an annular beam, a 100 kHz attosecond pulse train (APT) is reported with the highest energy so far (51 pJ/shot) on target (269 pJ at generation) among the high-repetition rate systems (>10 kHz) in which the attosecond pulses were temporally characterized. The on-target pulse energy is maximized by reducing the losses from the reflections and filtering of the high harmonics, and an unprecedented 19% transmission rate from the generation point to the target position is achieved. At the same time, the probe beam is also annular and low loss of this beam is reached by using another holey mirror to combine with the APT. The advantages of using an annular beam to generate attosecond pulses with a high-average power laser are demonstrated experimentally and theoretically. The effect of nonlinear propagation in the generation medium on the annular-beam generation concept is also analyzed in detail.

A novel method is suggested for temporal and spatial cleaning of high-brightness laser pulses, which seems more energy-scalable than that based on crossed polarizers and offers better contrast improvement compared to the plasma mirror technique. The suggested arrangement utilizes nonlinear modulation of the beam in the Fourier-plane leading both to directional and to temporal modulation. By the use of a ‘conjugate’ aperture arrangement before and after the nonlinear spatial selector, intensity dependent transmission is obtained; simultaneous temporal and spatial filtering can be realized both for amplitude and phase modulation. In the case of phase modulation introduced by plasma generation in noble gases the experimental observations are in good agreement with the theory; demonstrating  >103 improvement in the temporal contrast, ~40% throughput, associated with effective spatial filtering. Due to the broad spectral and power durability of the optical arrangement used here, the method is widely applicable for energetic beams even of UV wavelengths, where most of the former techniques have limited throughput.

It is demonstrated for the first time that plasma mirrors can be successfully applied for KrF laser systems. High reflectivity up to 70% is achieved by optimization of the beam quality on the plasma mirror. The modest spectral shift and the good reflected beam quality allow its applicability for high power laser systems for which a new arrangement is suggested.

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Recently a novel method called nonlinear Fourier-filtering was suggested for temporal and spatial cleaning of high-brightness laser pulses. In this paper experimental demonstration of the associated spatial filtering of this method and significant improvement of the temporal filtering feature are presented. The formerly found limit of ~10 for the temporal contrast improvement is identified as diffraction effects caused by the limited numerical aperture of imaging. It is shown by numerical simulation that proper apodization of the object can lead to sufficiently higher limit (>10). Using an advanced experimental arrangement the improvement of >2 orders of magnitude is experimentally verified in the ultraviolet and an indirect proof is presented that the background caused by the optical arrangement is reduced below 10.

The characteristics of terahertz (THz) radiation generated from large-aperture photoconductive antennas (LAPCAs) were investigated. The antennas were fabricated using different wide-bandgap semiconductor crystals (ZnSe, GaN, 6H-SiC, 4H-SiC and β-Ga 2 O 3 ) as the substrate. We used an amplified sub-picosecond KrF excimer laser for illumination of the LAPCAs. THz emission scaling was studied as a function of the bias field and the pump laser energy. It was found that the radiated THz energy scales quadratically as a function of the bias field and sub-linearly as a function of the optical fluence for most of the substrates. Further, we demonstrate that SiC, and especially 4H-SiC LAPCAs offer the best THz generation performances. In order to generate intense THz radiation, we fabricated both 6H-and 4H-SiC LAPCAs with an interdigitated structure. From the field autocorrelation trace, it was found that the spectra lie in the sub-THz regime, extending up to 400 GHz, with a peak frequency at 50 GHz, making the bridge between the microwaves band and the THz band. The maximum generated THz energy was 11 μJ, which is to date the highest THz energy measured from LAPCA sources, with a corresponding peak electric field of 115 kV cm −1 and a corresponding ponderomotive potential of 60 eV. Nonlinear THz experiments were performed using these energetic THz pulses, and open aperture Z-scan experiments in an n-doped InGaAs layer revealed a transmission enhancement of 1.7. It was also shown that in order to have efficient THz generation, the energy contrast of the laser must be kept high.

The energy and spectrum of the reflected 248 nm radiation are studied from solid targets up to 1.15 × 10 18  W cm −2 intensity. The experiments used the 700 fs directly amplified pulses of the KrF system which was cleaned from prepulses with the new Fourier-filtering method providing 12 orders of magnitude temporal contrast. Increasing the intensity from 10 15  W cm −2 results in increasing absorption up to more than 90% above 10 18  W cm −2 . This is accompanied by increasing x-ray conversion exhibiting a less steep power law dependence for low-Z matter than for gold. Strong blue shift of the reflected radiation from the backward propagating plasma was observed. It is shown that in the case of KrF laser pulses of highest contrast, vacuum heating can be one of the dominant absorption mechanisms. This article is part of a discussion meeting issue ‘Prospects for high gain inertial fusion energy (part 1)’.

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