Relativistic high-order harmonics from a few-cycle laser driven plasma surface is a very promising source of an intense and isolated attosecond light pulse. The laser to harmonics conversion efficiency and the “purity” of an isolated attosecond light pulse are generally determined by a combination of interaction parameters, such as laser intensities, incidence angles, pulse durations, carrier-envelope phases and plasma scale lengths. We had already previously investigated the effect of a three-parameter combination of the laser pulse duration, the carrier-envelope phase and the plasma scale length. To complement our previous work, the parametric dependence of the other two three-parameter combinations: the carrier-envelope phase, the plasma scale length, either combined with the laser intensity or the incidence angle, were systematically investigated through one dimensional particle-in-cell simulations. We found that, although the impact of parameter combinations on attosecond pulse generations is generally complicated, there exist however an optimal plasma scale length and an optimal incidence angle to efficiently generate high-order harmonics and intense attosecond light pulses. When other parameters are fixed, a moderately intense relativistic laser is more advantageous to realize an isolated attosecond light pulse in a broad controlling parameters range. And a larger incidence angle favors a higher isolation degree as well as a broader range of controlling parameters towards the generation of intense isolated attosecond light pulses. In order to interpret these simulation results, we have modeled the corresponding relativistic electron dynamics, using which the underlying physics are discussed.
Harmonics from relativistic laser driven plasma surfaces is a prospective high energy attosecond light source in future XUV pump-probe experiments. Among all the schemes, the most efficient and direct way to realize an isolated attosecond pulse is through using a few-cycle laser as the driving pulse. The two goodness criteria: the laser to harmonics energy conversion efficiency and the “purity” of an isolated attosecond pulse are generally determined by a combination of interaction parameters. Through using particle-in-cell simulations and relativistic electron dynamics model analyses, we explain how these two criteria are affected by the laser intensity, incidence angle, carrier-envelope phase, and the plasma scale length. We found that, there exist an optimal plasma scale length and an optimal incidence angle to efficiently generate harmonics and intense attosecond light pulses. When other parameters are fixed, using a moderately intense relativistic laser or using a large incidence angle could result in a higher isolation degree as well as a broader range of controlling parameters to realize an isolated attosecond light pulse.
A source of isolated attosecond pulses with photon energies lying in the water window soft x-ray range is currently under development at Deutsches Elektronen-Synchrotron. Such a source will be driven by the newly developed sub-cycle millijoule-level parametric waveform synthesizer. In this proceeding on theoretical study, in order to optimize the x-ray pulse energy while maintaining good pulse isolation in the soft x-ray range, a multi-objective genetic algorithm is exploited to tailor the laser electric field waveform. The resulting synthesized waveform are then employed in a macroscopic propagation study to predict x-ray pulse characteristics from a real experiment.
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