COL Cover Story: A new scheme of generating intense keV isolated attosecond pulses
At: 2017/6/30 17:35:00 by admin

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 easy to carry out, and is reliable.

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.

Graphic description: 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.

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