Liu Qingcao, Xu Yanxia, Qi Xin, Zhao Xiaoying, Ji Liangliang, Yu Tongpu, Wei Luo, Yang Lei, Hu Bitao
Opt Express. 2013 Dec 30;21(26):31925-39. doi: 10.1364/OE.21.031925.
Ultra-intense single attosecond pulse (AP) can be obtained from circularly polarized (CP) laser interacting with overdense plasma. High harmonics are naturally generated in the reflected laser pulses due to the laser-induced one-time drastic oscillation of the plasma boundary. Using two-dimensional (2D) planar particle-in-cell (PIC) simulations and analytical model, we show that multi-dimensional effects have great influence on the generation of AP. Self-focusing and defocusing phenomena occur in front of the compressed plasma boundary, which lead to the dispersion of the generated AP in the far field. We propose to control the reflected high harmonics by employing a density-modulated foil target (DMFT). When the target density distribution fits the laser intensity profile, the intensity of the attosecond pulse generated from the center part of the plasma has a flatten profile within the center range in the transverse direction. It is shown that a single 300 attosecond (1 as = 10(-18)s) pulse with the intensity of 1.4 × 10(21) W cm(-2) can be naturally generated. Further simulations reveal that the reflected high harmonics properties are highly related to the modulated density distribution and the phase offset between laser field and the carrier envelope. The emission direction of the AP generated from the plasma boundary can be controlled in a very wide range in front of the plasma surface by combining the DMFT and a suitable driving laser.
超短超强单阿秒脉冲（AP）可通过圆偏振（CP）激光与过密等离子体相互作用获得。由于激光诱导的等离子体边界的一次性剧烈振荡，在反射激光脉冲中自然会产生高次谐波。利用二维（2D）平面粒子模拟（PIC）和解析模型，我们表明多维效应对阿秒脉冲的产生有很大影响。在压缩等离子体边界前方会出现自聚焦和散焦现象，这会导致所产生的阿秒脉冲在远场中发生色散。我们建议采用密度调制箔靶（DMFT）来控制反射高次谐波。当靶密度分布与激光强度分布相匹配时，从等离子体中心部分产生的阿秒脉冲强度在横向中心范围内具有平坦的分布。结果表明，可以自然产生强度为1.4×10²¹W/cm²的单个300阿秒（1 as = 10⁻¹⁸s）脉冲。进一步的模拟表明，反射高次谐波特性与调制密度分布以及激光场与载波包络之间的相位偏移高度相关。通过结合DMFT和合适的驱动激光，可以在等离子体表面前方非常宽的范围内控制从等离子体边界产生的阿秒脉冲的发射方向。
