High-order harmonic generation in rare gases with an intense short-pulse laser

Wahlström, Claes‐Göran; Larsson, Jörgen; Persson, Anders; Starczewski, T.; Svanberg, Sune; Salieres, P.; Balcou, Philippe; L’Huillier, Anne

doi:10.1103/physreva.48.4709

Cited by 288 publications

(155 citation statements)

References 29 publications

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“…Amplitude modulations close to the position of the Cooper minimum have been observed in previous studies of argon high harmonic spectra using both 800 nm [13][14][15][16][17] and also longer mid-IR 2 µm fundamental wavelengths [18]. However, the observed amplitude minima have been addressed as a Cooper minumum only recently [19][20][21].…”

Section: Introductionmentioning

confidence: 86%

Influence of phase matching on the Cooper minimum in Ar high-order harmonic spectra

et al. 2011

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We systematically study the influence of phase matching on interference minima in high harmonic spectra. We concentrate on structures in atoms due to interference of different angular momentum channels during recombination. For this purpose, we use the Cooper minimum (CM) in argon at ∼ 47 eV as a marker in the harmonic spectrum. We measure two-dimensional harmonic spectra in argon as a function of wavelength and angular divergence. While we identify a clear CM in the spectrum when the target gas jet is placed after the laser focus, we find that the appearance of the CM varies with angular divergence and can even be completely washed out when the gas jet is placed closer to the focus. We also show that the argon CM appears at different wavelengths in harmonic and photo-absorption spectra measured under conditions independent of any wavelength calibration. We model the experiment with a simulation based on coupled solutions of the timedependent Schrödinger equation and the Maxwell wave equation, thereby including both the single atom response and macroscopic effects of propagation and phase matching. The single atom calculations confirm that the ground state of argon can be represented by its field free p symmetry, despite the strong laser field used in high harmonic generation. Because of this, the CM structure in the harmonic spectrum can be described as the interference of continuum s and d channels, whose relative phase jumps by π at the CM energy, resulting in a minimum shifted from the photoionization result. We also show that the full calculations reproduce the dependence of the CM on the macroscopic conditions. We calculate simple phase matching factors as a function of harmonic order and explain our experimental and theoretical observation in terms of the effect of phase matching on the shape of the harmonic spectrum. Our results emphasize that phase matching must be taken into account to fully understand spectral features related to harmonic spectroscopy. Furthermore, we show that in some cases phase matching can be actively used to enhance the visibility of interference minima in high harmonic spectra.2

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Section: Introductionmentioning

confidence: 86%

Influence of phase matching on the Cooper minimum in Ar high-order harmonic spectra

et al. 2011

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“…The Cooper minimum in Ar is usually observed in photoionization spectra [25]. It is also visible in high harmonic spectra [6,[26][27][28][29], but it has rarely recieved comment in the past [30]. Recent measurements have led to further discussion of the Cooper minimum in the HHG spectrum [31,32].…”

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confidence: 99%

High-order harmonic phase in molecular nitrogen

et al. 2009

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Electronic structure in atoms and molecules modulates the amplitude and phase of high harmonic generation (HHG). We report measurements of the high harmonic spectral amplitude and phase in N 2 . The phase is measured interferometrically by beating the N 2 harmonics with those of an Ar reference oscillator in a gas mixture. A rapid phase shift of 0.2π is observed in the vicinity of the HHG spectral minimum, where a shift of π had been presumed [J. Itatani et al., Nature 432, 867 (2004)]. We compare the phase measurements to a simulation of the HHG recombination step in N 2 that is based on a simple interference model. The results of the simulation suggest that modifications beyond the simple interference model are needed to explain HHG spectra in molecules.

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“…où A est une constante de normalisation, IO (t) est l'intensité du laser, q-effun ordre « effectif » de non-linéarité, supposé égal à 3 [13] et 1 g (o) = 1--Il + cos (20)] (2) 4 est une fonction qui exprime l'efficacité du spectromètre en fonction de l'orientation du plan de polarisation des harmoniques. La valeur de P, pour cette harmonique a été mesurée égale à 35.…”

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