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Abstract
Relativistic surface high harmonics, combined with the use of polarization gating, present a promising route towards intense single attosecond pulses. However, they impose stringent requirements on ultra-high laser contrast and are restricted by large intensity losses in real experiments. Here, we numerically demonstrate that by setting an optimal time delay in the polarization gating scheme, the intensity of the generated single attosecond pulses can become approximately 100 times stronger than that with nonoptimal time delay in the coherent synchrotron emission process. When a petawatt-class driving laser irradiates a solid target, an ultra-dense electron nanobunch and a strong space-charge sheath develop, and the accumulated electrostatic energy is only released in half of the laser cycle when this electron nanobunch moves backward. This process results in the emission of intense high harmonics. Our study provides a reliable method for developing bright attosecond extreme ultraviolet pulses.
Highlights
High-harmonic generation (HHG) from an ultra-intense laser solid-target interaction has been proven to be an important method for producing bright coherent extreme ultraviolet rays and X-rays [1,2,3]
Figure 6: 2D PIC simulation. (a) Intensity boost and modulation of harmonic spectrum in the polarization gating scheme. e intensity of HHG with optimal time delay can be increased by approximately 100 times when compared with the other nonoptimal time delay. e other simulation parameters are a 10, τ 6 T0, ne 100 nc, Ls 0.2 λL, and normal incidence. (b, c) Snapshots of electron density at different times when the time delay is 6 fs. e other simulation parameters are a 10, τ 6 T0, ne 100 nc, Ls 0.2 λL, and normal incidence
We have presented results from a large number of particlein-cell simulations, demonstrating that the polarization gating concept applied in relativistic surface high-harmonic generation can be used to overcome the limitation typically associated with the relatively long preplasma scale length
Summary
High-harmonic generation (HHG) from an ultra-intense laser solid-target interaction has been proven to be an important method for producing bright coherent extreme ultraviolet rays and X-rays [1,2,3]. The coherent wake emission (CWE) [3, 4, 6, 7] and relativistic oscillating mirror (ROM) [1, 8,9,10,11,12,13,14] models successfully describe the high-harmonics generation process at laser intensities of I < 1018W/cm and I > 1018W/cm, respectively. In recent years, nanobunching of relativistic electrons has been identified to be responsible for coherent synchrotron emission (CSE) [15,16,17,18]. In the CSE model, dense nanometer-scale bunches of electrons accelerated by the laser and plasma fields in relativistic trajectories radiate high-frequency light. For near-normal incidence, the plasma oscillations are driven by the Lorentz force of the incident laser pulse and charge-separation-induced electrostatic fields. In the case of the circularly polarized pulse, this force is of the form Fp ∼ a2(t). us, it exhibits no fast oscillations, creating a smooth density depression, with which no harmonics is allowed to generate
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