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Pressure dependence of high order harmonic generation in nitrogen molecular gas and atmospheric air
摘要： The effect of the variation of the gas pressure on the high harmonic generation (HHG) from nitrogen molecular gas (N2) and atmospheric air with ultrashort intense laser pulses is measured. The optimum pressure in the interaction region of a gas jet for maximizing the HHG yield is determined for both gases. Enhancement of the HHG output and its extension to higher harmonic orders are observed around the optimum pressure value of 0.33 bar. Theoretical calculations based on one-dimensional models explain this effect and provide reasonable agreement with experimental results.
关键词： Femtosecond laser,Conversion efficiency,High-order harmonic generation (HHG),Phase matching,XUV radiation,Pressure optimization
更新于2025-09-23 15:23:52
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High order harmonic generation in solids: a review on recent numerical methods
摘要： Interaction of intense lasers with solid materials offers an alternative way to achieve high-order harmonic generation (HHG). Since the underlying mechanisms of the harmonic emission remain uncertain, a large number of theoretical studies have been proposed to explain the experimental findings in this fast-growing field. Here, we review the primary numerical methods and present a brief perspective of HHG in solids.
关键词： Ultrafast laser,strong field,numerical simulations,laser-crystal interaction,solid materials,high-order harmonic generation
更新于2025-09-23 15:23:52
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Dynamical distortions of structural signatures in molecular high-order harmonic spectroscopy
摘要： We study the signature of two-center interferences in molecular high-order harmonic spectra, with an emphasis on the spectral phase. With the help of both ab initio computations based on the time-dependent Schr?dinger equation and the molecular strong-field approximation (SFA) as developed by Chiril? et al. [Phys. Rev. A 73, 023410 (2006)] and Faria [Phys. Rev. A 76, 043407 (2007)], we observe that the phase behavior is radically different for the short and the long trajectory contributions. By means of Taylor expansions of the molecular SFA, we link this effect to the dynamics of the electron in the continuum. More precisely, we find that the value of the electric field at recombination time plays a crucial role in the shape of the destructive interference phase jump.
关键词： molecular spectroscopy,spectral phase,strong-field approximation,two-center interferences,time-dependent Schr?dinger equation,high-order harmonic generation
更新于2025-09-23 15:23:52
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Enhancement of attosecond pulse in water window region by using pump-probe approach
摘要： An effective scheme to enhance the harmonic yield and the attosecond (as) pulse signal from H+2 has been proposed by using the pump-probe laser field. It is found that with the help of the multi-cycle pump pulse, H+2 can be steered into the resonance ionization regions, where the ionization probability can be remarkably enhanced. Thus, the harmonic yield can be remarkably enhanced when a sequential mid-infrared probe pulse is added. Further, by modulating the phase of the probe pulse and by adding a controlling pulse, the harmonic cutoff can be extended up to the water window region. Finally, by the Fourier transformation of the selected harmonics on the harmonic supercontinuum, the water window attosecond pulses as short as 25 as can be obtained.
关键词： High-order harmonic generation,pump-probe pulse,charge resonance enhanced ionization,single attosecond pulse,harmonic enhancement
更新于2025-09-23 15:21:21
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Attosecond pulse generation via multicolor beam superposition in the over-barrier ionization regime
摘要： Based on coherent superposition of multicolor continuous wave beams, high order harmonics and attosecond pulse generation from a hydrogen molecular ion (HMI), +H ,2 with different internuclear distances, R, are studied. Driving pulse intensity was taken to be 400 TW cm?2, which can ionize an +H2 ion with R?>?Rb?=?2.4 a.u. over the barrier. It is seen that cutoff harmonic with respect to R decreases until the over barrier ionization threshold, Rb, then becomes almost uniform to the end of R?=?10 a.u. whereas it is almost constant with respect to the spectral width of the beams, Δω. Besides attosecond pulse intensity increases versus R, and decreases with respect to Δω so that the most intense attosecond pulses can be generated from HMIs with R???5 a.u. via the pulses by Δω???0.03 a.u. Attosecond pulse durations were insensitive to both Δω and R.
关键词： hydrogen ion molecule,high order harmonic generation,H2,attosecond
更新于2025-09-23 15:21:21
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Coherent Tabletop EUV Ptychography of Nanopatterns
摘要： Coherent diffraction imaging (CDI) or lensless X-ray microscopy has become of great interest for high spatial resolution imaging of, e.g., nanostructures and biological specimens. There is no optics required in between an object and a detector, because the object can be fully recovered from its far-field diffraction pattern with an iterative phase retrieval algorithm. Hence, in principle, a sub-wavelength spatial resolution could be achieved in a high-numerical aperture configuration. With the advances of ultrafast laser technology, high photon flux tabletop Extreme Ultraviolet (EUV) sources based on the high-order harmonic generation (HHG) have become available to small-scale laboratories. In this study, we report on a newly established high photon flux and highly monochromatic 30 nm HHG beamline. Furthermore, we applied ptychography, a scanning CDI version, to probe a nearly periodic nanopattern with the tabletop EUV source. A wide-field view of about 15 × 15 μm was probed with a 2.5 μm?diameter illumination beam at 30 nm. From a set of hundreds of far-field diffraction patterns recorded for different adjacent positions of the object, both the object and the illumination beams were successfully reconstructed with the extended ptychographical iterative engine. By investigating the phase retrieval transfer function, a diffraction-limited resolution of reconstruction of about 32 nm is obtained.
关键词： high-order harmonic generation,ptychography,lensless X-ray microscopy,Extreme Ultraviolet,Coherent diffraction imaging
更新于2025-09-23 15:21:21
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Semi-integer harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized laser field
摘要： We theoretically investigated the properties of the high-order harmonic generation from an argon atom by bichromatic counter-rotating circularly polarized (BCCP) laser field. The harmonic spectra are gradually splitting and the harmonic orders move towards the higher frequency. Meanwhile, the integer and semi-integer harmonic emission will be generated when the frequency ratios of right- and left-circularly polarized laser are semi-integer. The emission mechanism of the semi-integer-order harmonics have been investigated by using the rules of photon absorption and emission.
关键词： bichromatic counter-rotating circularly polarized laser field,photon absorption and emission,high-order harmonic generation,semi-integer-order
更新于2025-09-23 15:21:01
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Cooper minimum of high-order harmonic spectra from an MgO crystal in an ultrashort laser pulse
摘要： Cooper minimum structure of high-order harmonic spectra from atoms or molecules has been extensively studied. In this paper, we demonstrate that the crystal harmonic spectra from an ultrashort midinfrared laser pulse also exhibit the Cooper minimum characteristic. Based on the accurate band dispersion and k-dependent transition dipole moment (TDM) from the ?rst-principles calculations, it can be found that the harmonic spectra from the MgO crystal along the (cid:2)-X direction present a dip structure in the plateau, which is originated from the valley of TDM by examining the distribution of the harmonic intensity at the k space. The Cooper minimum feature in crystal high-order-harmonic generation (HHG) will pave a new way to retrieve the band information of solid materials by using HHG from the ultrashort midinfrared laser pulse.
关键词： transition dipole moment,ultrashort laser pulse,Cooper minimum,MgO crystal,high-order harmonic generation
更新于2025-09-23 15:21:01
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Influence of intensity ratio of the two driving laser fields on the cutoff of the high-order harmonics
摘要： We investigate the high-order harmonic generation (HHG) of the helium atom at the various intensity ratios of the counter-rotating two-color circularly polarized laser fields. The results show that the cutoff energy of the harmonic spectra can be affected by changing the intensity ratios. By comparing the results from different intensity ratios, we find that the distances the electrons travel are short when the intensity ratio is increased. The dependence of the cutoff position in the HHG on the intensity ratios usually results from the classical trajectories of the electrons. As the intensity ratio increases, the electrons cannot be accelerated and it will gain less energy. As a result, the harmonic spectra are cut off at the lower energy. We compare the quantum and classical trajectories results to illustrate the reasons of the cut off dependence.
关键词： intensity ratios,cutoff energy,high-order harmonic generation,two-colour circularly polarized laser fields
更新于2025-09-23 15:21:01
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Ellipticity control of high-order harmonic generation with nearly orthogonal two-color laser fields
摘要： Elliptically polarized high-order harmonics opens a path toward the study of ultrafast dynamics in circular dichroism of magnetic materials, chiral molecules, and electronic spin motion. Recently, efficient high-order harmonic generation with controllable ellipticity has attracted considerable attention in experiment and theory. Here, we experimentally demonstrate a scenario using cross-linearly-polarized two-color fields consisting of a fundamental field and its second harmonic (SH). Through adjusting the relative phase, crossing angle, and intensity ratio of the cross-linearly-polarized two-color field, we obtain ellipticity-tunable high-order harmonics, with ellipticity up to ε = 0.73. Meanwhile, the elliptically polarized high-order harmonics preserve high efficiency. Furthermore, when the intensity of the SH field is stronger than that of the fundamental field, the even order harmonics will become several times stronger than the odd order harmonics with opposite helicity.
关键词： attosecond science,high-order harmonic generation,ellipticity control,strong-field physics,two-color laser fields
更新于2025-09-23 15:21:01