A new scheme of generating intense keV isolated attosecond pulses
High harmonic spectrum (top left) and its time-frequency map (bottom left) generated by the 1800 nm/900 nm MIR-OTC scheme and corresponding time profiles of isolated attosecond pulses by superposing different spectral humps (right). ω0 and T0 are the angular frequency and optical cycle of the 1800 nm fundamental pulse, respectively.
The generation of intense isolated attosecond pulses (IAPs) covering the keV regime has become a hot topic at present due to their practical applications such as biological imaging, X-ray diffraction, attosecond pump-probe dynamics. High-order harmonic generation (HHG) is one of the most powerful approaches to generate attosecond pulses.
Generally, mid-infrared (MIR) lasers with the wavelength up to several micrometers have been extensively used as the driving sources for keV high-order harmonic generation (HHG) because of the prediction of the single-atom cut-off law that the maximum photon energy is proportional to the driver wavelength. Unfortunately, the high harmonic efficiency will drop dramatically with increasing wavelength, with a very unfavorable λ-(5~6) scaling, which severely hinders its practical applications.
To improve the high harmonic efficiency, a new scheme for intense keV isolated attosecond pulse (IAP) generation by orthogonally polarized multicycle mid-infrared two-color (MIR-OTC) laser fields has been proposed by Professor Zhinan Zeng and Yinghui Zheng's research group, from Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences. The obtained IAPs are enhanced by 30 times and 5 times in magnitude, compared with the results obtained by single-color and parallel two-color schemes. The research result is reported in Chinese Optics Letters, Volume 15, No. 7, 2017 (Guicun Li, et al., Intense keV isolated attosecond pulse generation by orthogonally polarized multicycle mid-infrared two-color laser fields).
Based on the waveform control method, the scheme consists of a multicycle mid-infrared laser field and its orthogonally polarized second harmonic laser field to interact with the helium atom. By optimizing the two-color delay, multiple humps with different central photon energies can be obtained in the MIR-OTC field, due to the 2D manipulation of the electron-ion recollision process and the suppression of inter-half-cycle interference effect. By filtering these humps, about 360 as, up to 1.8 keV intense isolated attosecond pulses are directly generated without any phase compensation in the 4 µm/2 µm OTC field. The scheme is reliable and easy to carry out.
The MIR-OTC scheme, combined with the loosely focused conditions, can be used for further increasing the energy of the IAPs. The following work will focus on the experimental realization of the loosely focused MIR-OTC scheme for intense keV IAP generation by directly using multicycle TW or even PW mid-infrared lasers.
高强度keV单阿秒脉冲产生新方案
图片说明：利用多光周期1800 nm/900 nm中红外垂直偏振双色场方案得到高次谐波谱光谱。左上图为高次谐波谱，左下图为时频分析图；右图为选取不同的光谱“驼峰”对应时域上产生的单阿秒脉冲。图中ω0，T0分别是1800 nm基频光的角频率和光周期。
高强度的keV单阿秒脉冲（IAP）以其在物理、材料和信息技术等众多领域存在广泛应用，近年来成为研究热点。阿秒脉冲的产生方法有许多种，其中最有效的方法之一是通过生成高次谐波来得到。高次谐波截止区光子能量与驱动波长的平方成正比，因此实验上通常用波长为数微米的中红外激光作为驱动光源来产生keV的高次谐波。然而，由于转换效率以波长的-5到-6次方形式急剧下降，因此通过高次谐波方法产生的单阿秒脉冲强度很低，极大地限制了其应用范围。
为克服keV单阿秒脉冲强度低的问题，中国科学院上海光学精密机械研究所曾志男、郑颖辉研究员的课题组提出了利用多光周期中红外垂直偏振双色场（MIR-OTC）产生高强度keV单阿秒脉冲的方案。模拟结果发现，利用这一方案产生的阿秒脉冲强度相比于传统方法有较大的提高，强度比单色场方案提高30倍，比平行偏振双色场方案提高5倍。相关结果发表在Chinese Optics Letters 2017年第7期上（Guicun Li, et al., Intense keV isolated attosecond pulse generation by orthogonally polarized multicycle midinfrared two-color laser fields）。
该方案基于光场整形原理，利用多光周期中红外激光和与之垂直偏振的倍频光与氦气相互作用，通过改变双色场延时，可以在二维方向操控电子与母核的碰撞过程，大幅减少了轨道间的干涉效应，由此得到多个不同光谱段的“驼峰”状的准连续谱。通过这一方案，无需相位补偿，直接滤出4 µm/2 µm MIR-OTC方案产生的“驼峰”，可以得到高强度的1.8 keV，约360 as的脉冲。
该工作还指出，利用多光周期驱动脉冲，结合长聚焦方案，允许更高的抽运能量，可以得到更强的keV单阿秒脉冲。相比于传统方法，该方案简单易实现，且更可靠。
后续工作将讨论宏观传播对该方案的影响，并直接利用多光周期太瓦或者拍瓦中红外垂直偏振双色场，结合长聚焦方案，从实验上产生高强度的keV单阿秒脉冲。