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    Effect of laser intensity on quantum trajectories in the macroscopic high-order harmonic generation
    Yun Pan(潘云), Fuming Guo(郭福明), Yujun Yang(杨玉军), Dajun Ding(丁大军)
    Chin. Phys. B, 2019, 28 (11): 113201.   DOI: 10.1088/1674-1056/ab47f8
    Abstract377)   HTML    PDF (625KB)(167)      
    We macroscopically investigate the effect of the laser intensity and gas density on quantum trajectories in the high-order harmonic generation of Ne atoms irradiated by few-cycle, 800-nm laser pulses. The time-frequency profile of the harmonics shows that the long quantum trajectory is dominant at both lower and higher gas densities for a low laser intensity. At high laser intensities, the long quantum trajectory plays an important role for lower gas densities, while the short quantum trajectory is dominant at higher gas densities. An analysis of the phase mismatch for high-order harmonic generation shows that the primary emission of the quantum trajectories is determined by dynamic changes in the laser electric field during the propagation process.
    Role of Bloch oscillation in high-order harmonic generation from periodic structure
    Lu Liu(刘璐), Jing Zhao(赵晶), Jian-Min Yuan(袁建民), Zeng-Xiu Zhao(赵增秀)
    Chin. Phys. B, 2019, 28 (11): 114205.   DOI: 10.1088/1674-1056/ab47f7
    Abstract406)   HTML    PDF (2819KB)(159)      
    The high-order harmonic generation from a model solid structure driven by an intense laser pulse is investigated using the semiconductor Bloch equations (SBEs). The main features of harmonic spectrum from SBEs agree well with the result of the time-dependent Schrödinger equation (TDSE), and the cut-off energy can be precisely estimated by the recollision model. With increasing the field strength, the harmonic spectrum shows an extra plateau. Based on the temporal population of electron and the time-frequency analysis, the harmonics in the extra plateau are generated by the Bloch oscillation. Due to the ultrafast time response of the Bloch electron, the generated harmonics provide a potential source of shorter isolated attosecond pulse.
    Observation of the optical X2Σg+-A2Πu coupling in N2+ lasing induced by intense laser field
    Siqi Wang(王思琪), Yao Fu(付尧), Danwen Yao(姚丹雯), Shanming Chen(陈善铭), Wei Zhang(张维), Helong Li(李贺龙), Huailiang Xu(徐淮良)
    Chin. Phys. B, 2019, 28 (12): 123301.   DOI: 10.1088/1674-1056/ab54b1
    Abstract280)   HTML    PDF (637KB)(142)      
    We propose a simple pump-coupling-seed scheme to examine the optical X2Σg+-A2Πu coupling in N2+ lasing. We produce the N2+ lasing at 391 nm, corresponding to the B2Σu+(v'=0)-X2Σg+(v=0) transition, by externally seeding the N2+ gain medium prepared by irradiation of N2 with an intense pump pulse. We then adopt a weak coupling pulse in between the pump and seed pulses, and show that the intensity of the 391-nm lasing can be efficiently modulated by varying the polarization direction of the coupling pulse with respect to that of the pump pulse. It is found that when the polarization directions of the pump and coupling pulses are perpendicular, the 391-nm lasing intensity is more sensitive to the coupling laser energy, which reflects the inherent nature of the perpendicular X2Σg+-A2Πu transition.
    Quantum interference of multi-orbital effects in high-harmonic spectra from aligned carbon dioxide and nitrous oxide
    Hong-Jing Liang(梁红静), Xin Fan(范鑫), Shuang Feng(冯爽), Li-Yu Shan(单立宇), Qing-Hua Gao(高庆华), Bo Yan(闫博), Ri Ma(马日), Hai-Feng Xu(徐海峰)
    Chin. Phys. B, 2019, 28 (9): 094207.   DOI: 10.1088/1674-1056/ab3438
    Abstract336)   HTML    PDF (607KB)(198)      

    We investigate experimentally multi-orbital effects in high-order harmonic generation (HHG) from aligned CO2 and N2O molecules by intense femtosecond laser fields with linear and elliptical polarizations. For either of the aligned molecules, a minimum in the harmonic spectrum is observed, the position of which shifts to lower-order harmonics when decreasing the intensity or increasing the ellipticity of the driving laser. This indicates that the minimum originates from the dynamic interference of different channels, of which the tunneling ionization and recombination are contributed via different molecular orbitals. The results show that both the highest occupied molecular orbital (HOMO) and low-lying HOMO-2 in CO2 (or HOMO-1 in N2O) contribute to the molecular HHG in both linearly and elliptically polarized strong laser fields. Our study would pave a way for understanding multi-electron dynamics from polyatomic molecules irradiated by strong laser fields.

    Trajectory analysis of few-cycle strong field ionization in two-color circularly polarized fields
    Yan Huang(黄燕), Chaochao Qin(秦朝朝), Yizhu Zhang(张逸竹), Xincheng Wang(王新成), Tian-Min Yan(阎天民), Yuhai Jiang(江玉海)
    Chin. Phys. B, 2019, 28 (9): 093202.   DOI: 10.1088/1674-1056/ab3444
    Abstract507)   HTML    PDF (1389KB)(326)      

    Bichromatic circularly polarized fields provide a useful tool to probe the ionization dynamics. In this work, we compare the photoelectron momentum distribution in few-cycle bichromatic field of different helicities. The spectral features are analyzed with semiclassical trajectories derived from the strong field approximation. In particular, the interference fringes in momentum distribution are investigated by tracking the ionization time and tunneling exits of released photoelectrons. Different types of trajectories that contribute to the interference fringes are elucidated.

    Coulomb exploded directional double ionization of N2O molecules in multicycle femtosecond laser pulses
    Junyang Ma(马俊杨), Kang Lin(林康), Qinying Ji(季琴颖), Wenbin Zhang(张文斌), Hanxiao Li(李韩笑), Fenghao Sun(孙烽豪), Junjie Qiang(强俊杰), Peifen Lu(陆培芬), Hui Li(李辉), Xiaochun Gong(宫晓春), Jian Wu(吴健)
    Chin. Phys. B, 2019, 28 (9): 093301.   DOI: 10.1088/1674-1056/ab38aa
    Abstract369)   HTML    PDF (1828KB)(189)      

    We experimentally investigate Coulomb exploded directional double ionization of N2O molecules in elliptically polarized femtosecond laser pulses. The denitrogenation and deoxygenation channels are accessed via various pathways. It leads to distinct asymmetries in directional breaking of the doubly ionized N2O molecules versus the instantaneous laser field vector, which is revealed by tracing the sum-momentum spectra of the ionic fragments as a recoil of the ejected electrons. Our results demonstrate that the accessibility of the Coulomb exploded double ionization channels of N2O molecules are ruled by the detailed potential energy curves, and the directional emission of the fragments are governed by the joint effects of the electron localization-assisted enhanced ionization of the stretched molecules and the profiles of the molecular orbitals.