Abstract
The influence of plasma density gradients on the harmonic generation process from relativistically oscillating mirror (ROM) is studied experimentally. It is observed that the harmonic intensities from orders of 21st to 24th first drop and then reach the maximum value with the increase of plasma scale length L. 2D particle-in-cell simulations are performed to investigate the intensities of high-order harmonics at different L values, which show that there are two optimal scale lengths Lopt for efficient ROM harmonic generation. The two optimal Lopt values are interpreted by a quasi-1D analytical model. By matching the potential energies provided by laser pulse and charge displacement in the plasma with an exponential distributed density profile, the model can quantitatively predict the optimal Lopt at the given laser incidence angle and intensity. Our work is beneficial for better understanding the role of L in ROM harmonic generation and the parameters affecting the optimal Lopt.
Highlights
High-order harmonic generation (HHG) by reflection of intense laser pulses from overdense plasma surfaces has been a subject of considerable interest in recent decades.[1,2,3] The main motivation that drives such a field is to obtain coherent extreme ultraviolet (EUV) or x-ray radiation.[4,5] In the time domain, the superposition of harmonics leads to the generation of attosecond pulse trains.[6]
By matching the potential energies provided by laser pulse and charge displacement in the plasma with an exponential distributed density profile, the two optimal Lopt values can be quantitatively estimated for efficient relativistically oscillating mirror (ROM) harmonic generation at the given laser incidence angle and intensity
The harmonic intensities are minimal at Lopt 1⁄4 0:11k. 2D PIC simulations are performed, which show that two optimal Lopt values are found, one optimal value at Llopt 1⁄4 0:07k, while the other optimal value at Luopt 1⁄4 0:2k
Summary
High-order harmonic generation (HHG) by reflection of intense laser pulses from overdense plasma surfaces has been a subject of considerable interest in recent decades.[1,2,3] The main motivation that drives such a field is to obtain coherent extreme ultraviolet (EUV) or x-ray radiation.[4,5] In the time domain, the superposition of harmonics leads to the generation of attosecond (as) pulse trains.[6]. Et al continuously tuned L in the range of 0 < L < 0:15k to investigate the role of L experimentally.[27] They demonstrate that there is an optimal scale length Lopt in the range of 0:05k < L < 0:1k for the ROM harmonics. We experimentally investigate the effect of the plasma density gradient on harmonic generation dominated by the ROM mechanism. By matching the potential energies provided by laser pulse and charge displacement in the plasma with an exponential distributed density profile, the two optimal Lopt values can be quantitatively estimated for efficient ROM harmonic generation at the given laser incidence angle and intensity
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