High-order harmonic generation by chirped and self-guided femtosecond laser pulses. I. Spatial and spectral analysis

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 harmonic field propagation in a macroscopic medium is described by a 3D Maxwell equation [28][29][30][31] …”

Section: Theoretical Methodsmentioning

confidence: 99%

Effects of macroscopic propagation on spectra of broadband supercontinuum harmonics and isolated-attosecond-pulse generation: Coherent control of the electron quantum trajectories in two-color laser fields

Chu

2012

Phys. Rev. A

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Recently it was shown that broadband supercontinuum harmonics can be produced from the long-trajectory electrons in the single-atom response by the coherent control of the electron trajectories through optimized two-color laser fields. Such supercontinuum harmonics can be superposed to generate an isolated sub-30-attosecond (as) pulse [Liu et al., Phys. Rev. A 84, 033414 (2011)]. In this paper, we investigate the effect of macroscopic propagation on the supercontinuum harmonic spectra and the subsequent attosecond-pulse generation of atomic hydrogen. The time-dependent Schrödinger equation is solved accurately and efficiently by means of the time-dependent generalized pseudospectral method. The effects of macroscopic propagation are investigated in near and far field by solving Maxwell's equation. The results show that the contribution of short-trajectory electron emission is increased when the macroscopic propagation is considered. However, the characteristics of the dominant long-trajectory electron emission (in the single-atom response case) are not changed, and an isolated 53 as pulse can be generated in the near field. Moreover, in the far field, the contribution of long-trajectory electron emission is still dominant for both on-axis and off-axis cases. As a result, an isolated 42 as pulse can be generated directly. Similar results are obtained when the atomic target position is changed. Therefore, the proposed method for the single ultrashort attosecond-pulse generation can be realized by means of the coherent control of the electron quantum paths in appropriately optimized two-color laser fields.

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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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High-order harmonic generation by chirped and self-guided femtosecond laser pulses. I. Spatial and spectral analysis

Cited by 86 publications

References 20 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

Effects of macroscopic propagation on spectra of broadband supercontinuum harmonics and isolated-attosecond-pulse generation: Coherent control of the electron quantum trajectories in two-color laser fields

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

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