For experiments in plasma, nuclear, and high-energy physics, there is a strong demand for laser pulses exhibiting relativistic intensity, few-cycle pulse duration, and a very high contrast. Here we present a picosecond-pumped optical parametric chirped pulse amplification (OPCPA) system delivering pulses at 10 Hz repetition rate with the following key parameters: a compressed pulse duration of less than 7 fs (close to the Fourier limit), a contrast of better than 10 11 starting from 1 ps before the main pulse, and a peak intensity of 6.9 × 10 19 W∕cm 2 achieved with an off-axis parabolic mirror (f/1.6). In a proof-of-principle experiment, these pulses were used to generate high harmonics from solid surfaces with photon energies exceeding 55 eV. These results underline the promising perspectives of the reported system for relativistic light-matter interaction experiments and attosecond science.
Relativistic surface high harmonics have been considered a unique source for the generation of intense isolated attosecond pulses in the extreme ultra-violet (XUV) and X-ray spectral range. However, its experimental realization is still a challenging task requiring the identification of the optimum conditions for the generation of isolated attosecond pulses as well as their temporal characterization. Here, we demonstrate measurements in both directions. Particularly, we have made a first step towards the temporal characterization of the emitted XUV radiation by adapting the attosecond streak camera concept to identify the time domain characteristics of relativistic surface high harmonics. The results, supported by PIC simulations, set the upper limit for the averaged (over many shots) XUV duration to < 7 fs, even when driven by not CEP controlled relativistic few-cycle optical pulses. Moreover, by measuring the dependence of the spectrum of the relativistic surface high harmonics on the carrier envelope phase (CEP) of the driving infrared laser field, we experimentally determined the optimum conditions for the generation of intense isolated attosecond pulses.The invention of sources of attosecond pulses based on high-order harmonic generation (HHG) [1-3] has opened the field of attosecond science [4,5] with a wide range of potential applications [6]. Nowadays, attosecond science is mainly based on the HHG in gas media which allows the generation of isolated attosecond pulses on the nanoto few-micro Joule energy level with photon energies up to sub-keV. However, this approach has fundamental limitations determined by the ionization threshold of the gas medium [4,7], leading to severe restrictions on the XUV flux especially at high photon energies.A way to overcome this limitation is to use relativistic harmonics generated by interaction of intense fewcycle laser fields with solid surfaces [8][9][10][11]. Theoretical predictions, based on the relativistic oscillating mirror (ROM) model [9], have suggested that intense isolated attosecond pulses with up to few keV photon energy can be generated when using few-cycle near-infrared (NIR) laser pulses with an intensity of ∼ 10 20 W/cm 2 . Therefore, ROM harmonics present one of the most promising attosecond sources for pump-probe studies in the X-ray spectral range. Yet, experimental obstacles associated mainly with the stringent requirements on the temporal contrast of the driving laser pulses have not yet allowed sufficient progress to realize the potential of this approach. However, recent progress in the development of laser systems based on optical parametric chirped pulse amplification (OPCPA) with pump pulse durations between 1 ps [12] and 80 ps [13] made the required pulse parameters available. Although the generation of isolated attosecond pulses from relativistic laser-plasma interactions driven by few-cycle optical pulses has been theoretically predicted using one-dimensional particle in cell (1D-PIC) simulations [11,14], its experimental realization remains ...
High-field experiments are very sensitive to the exact value of the peak intensity of an optical pulse due to the nonlinearity of the underlying processes. Therefore, precise knowledge of the pulse intensity, which is mainly limited by the accuracy of the temporal characterization, is a key prerequisite for the correct interpretation of experimental data. While the detection of energy and spatial profile is well established, the unambiguous temporal characterization of intense optical pulses, another important parameter required for intensity evaluation, remains a challenge, especially at relativistic intensities and a few-cycle pulse duration. Here, we report on the progress in the temporal characterization of intense laser pulses and present the relativistic surface second harmonic generation dispersion scan (RSSHG-D-scan)—a new approach allowing direct on-target temporal characterization of high-energy, few-cycle optical pulses at relativistic intensity.
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