Nonadiabatic three-dimensional model of high-order harmonic generation in the few-optical-cycle regime

We report measurements and theoretical simulations of high-order harmonic generation (HHG) in aligned N 2 molecules using a 1200-nm intense laser field when the generating pulse is perpendicular to the aligning one. With increasing laser intensity, the minimum in the HHG spectra first shifts its position and then disappears. Theoretical simulations including the macroscopic propagation effects in the medium reproduce these observations and the disappearance of the minimum is attributed to the additional contribution of HHG from inner orbitals. We also predict that the well-known shape resonance in the photoionization spectra of N 2 should exist in the HHG spectra. It is most clearly seen when the generating laser is parallel to the aligning one and disappears gradually as the angle between the two lasers increases. No clear evidence of this shape resonance has been reported so far when using lasers with different wavelengths. Further experimentation is needed to draw conclusions.

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“…The three-dimensional Maxwell's wave equation for the xuv light in the gas medium is [24,[34][35][36][37]]…”

Section: Theoretical Methodsmentioning

confidence: 99%

Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N2molecules

et al. 2012

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“…The 3-D propagation equation of the harmonic field is described by [16,107,114] 48) where P (r, z, t) is the polarization depending upon the applied optical field E 1 (r, z, t). In this equation, the free-electron dispersion is neglected because the frequencies of high harmonics are much higher than the plasma frequency.…”

Section: High-harmonic Field Of Atomsmentioning

confidence: 99%

Theory of Nonlinear Propagation of High Harmonics Generated in a Gaseous Medium

Jin

2013

Springer Theses

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In this thesis, we establish the theoretical tools to investigate high-order harmonic generation (HHG) by intense infrared lasers in a gaseous medium. The macroscopic propagation of both the fundamental and the harmonic fields is taken into account by solving Maxwell's wave equations, while the single-atom (or single-molecule) response is obtained by quantitative rescattering theory. The initial spatial mode of the fundamental laser is assumed either a Gaussian or a truncated Bessel beam. On the examples of Ar, N 2 and CO 2 , we demonstrate that the available experimental HHG spectra with isotropic and aligned target media can be accurately reproduced theoretically even though the HHG spectra are sensitive to the experimental conditions. We show that the macroscopic HHG can be expressed as a product of a macroscopic wave packet and a photorecombination cross section, where the former depends on laser and experimental conditions while the latter is the property of the target only. The factorization makes it possible to retrieve the single-atom or single-molecule structure information from experimental HHG spectra. As for the multiple molecular orbital contribution in HHG, it causes the disappearance of the minimum in the HHG spectrum of aligned N 2 with the increase of laser intensity, and the position of minimum in HHG spectrum of aligned CO 2 depending on many factors is also attributed to it, which could explain why the minima observed in different laboratories may differ. For an important application of HHG as ultrashort light source, we show that measured continuous harmonic spectrum of Xe due to the reshaping of the fundamental laser field can be used to produce an isolated attosecond pulse by spectral and spatial filtering in the far field. For on-going application of using HHG to ionize aligned molecules, we present the photoelectron angular distribution from aligned N 2 and CO 2 in the laboratory frame, which can be compared directly with future experiments. demonstrate that the available experimental HHG spectra with isotropic and aligned target media can be accurately reproduced theoretically even though the HHG spectra are sensitive to the experimental conditions. We show that the macroscopic HHG can be expressed as a product of a macroscopic wave packet and a photorecombination cross section, where the former depends on laser and experimental conditions while the latter is the property of the target only. The factorization makes it possible to retrieve the single-atom or single-molecule structure information from experimental HHG spectra. As for the multiple molecular orbital contribution in HHG, it causes the disappearance of the minimum in the HHG spectrum of aligned N 2 with the increase of laser intensity, and the position of minimum in HHG spectrum of aligned CO 2 depending on many factors is also attributed to it, which could explain why the minima observed in different laboratories may differ. For an important application of HHG as ultrashort light source, we show that measured continu...

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“…Because of their excellent spatial and temporal coherence properties, highharmonic light sources have been widely applied in different research areas of physics and chemistry [3], from the probing of ultrafast electronic processes [4,5] to ultrahigh precision measurement of narrow XUV transitions [6], imaging of nanoscale structure [7], and so on. High-order harmonic generation (HHG) is a highly nonlinear process which is sensitive to the driving laser and the gas target [8][9][10][11]. Therefore, there is a wide range of macroscopic parameters that affects the generation of harmonics in an experiment, thus making it difficult to locate the needed phase-matching conditions for the efficient buildup of macroscopic harmonic fields in a gas medium [12][13][14].…”

Section: Introductionmentioning

confidence: 99%

Macroscopic scaling of high-order harmonics generated by two-color optimized waveforms in a hollow waveguide

2017

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We present the macroscopic scaling of high harmonics generated by two-color laser pulses interacting with Ne gas in a hollow waveguide. We demonstrate that the divergence of harmonics is inversely proportional to the waveguide radius and harmonic yields are proportional to the square of the waveguide radius when the gas pressure and waveguide length are chosen to meet the phase-matching condition. We also show that harmonic yields are inversely proportional to the ionization level of the optimized two-color waveform with proper gas pressure if waveguide radius and length are fixed. These scaling relations would help experimentalists find phase-matching conditions to efficiently generate tabletop high-flux coherent soft x rays for applications.

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Nonadiabatic three-dimensional model of high-order harmonic generation in the few-optical-cycle regime

Cited by 246 publications

References 37 publications

Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N2molecules

Intensity dependence of multiple orbital contributions and shape resonance in high-order harmonic generation of aligned N2molecules

Theory of Nonlinear Propagation of High Harmonics Generated in a Gaseous Medium

Macroscopic scaling of high-order harmonics generated by two-color optimized waveforms in a hollow waveguide

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