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.
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。