<b><i>Colloquium</i></b>: Optimal control of high-harmonic generation

Winterfeldt, Carsten; Spielmann, Christian; Gerber, G.

doi:10.1103/revmodphys.80.117

Cited by 366 publications

(248 citation statements)

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“…Here we present the nonlinear frequency conversion of a near-infrared laser beam carrying a phase singularity into the XUV by HHG. It should be emphasized that, in contrast to previous wavelength conversion experiments relying on perturbative nonlinear optics, HHG is a non-perturbative nonlinear process 14,15,17,18 .The experiments are performed with a sub-30 fs laser system. With a reflective spatial light modulator (SLM) the helical phase is imprinted onto the laser beam, before focusing it into an argon target for HHG.…”

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confidence: 99%

“…More recently the wavelength range of ultrashort pulses has been extended into the XUV region by nonlinear frequency conversion, termed as high-harmonic generation 17 (HHG). HHG is microscopically described by tunnel ionization, followed by propagation of the free electron in the laser field and finally recombination with the parent ion, accompanied by the emission of an XUV photon.…”

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confidence: 99%

“…The lower integrated yield is explained by the strong intensity dependence 19 (roughly proportional to I 6 ) of the conversion efficiency. The missing parts of the XUV beam are attributed to a lack of phase matching 15,17 and the non-uniform intensity of the fundamental laser beam. Replacing the CCD with an imaging spectrometer, we checked whether the diameter of the doughnut depends on the harmonic order.…”

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Strong-field physics with singular light beams

et al. 2012

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Light beams carrying a point singularity with a screw-type phase distribution are associated with an optical vortex. The corresponding momentum flow leads to an orbital angular momentum of the photons 1-3 . The study of optical vortices has led to applications such as particle micro-manipulation 4,5 , imaging 6 , interferometry 7 , quantum information 8 and highresolution microscopy and lithography 9 . Recent analyses showed that transitions forbidden by selection rules seem to be allowed when using optical vortex beams 10 . To exploit these intriguing new applications, it is often necessary to shorten the wavelength by nonlinear frequency conversion. However, during the conversion the optical vortices tend to break up 11-13 . Here we show that optical vortices can be generated in the extreme ultraviolet (XUV) region using high-harmonic generation 14,15 . The singularity impressed on the fundamental beam survives the highly nonlinear process. Vortices in the XUV region have the same phase distribution as the driving field, which is in contradiction to previous findings 16 , where multiplication of the momentum by the harmonic order is expected. This approach opens the way for several applications based on vortex beams in the XUV region.Places where physical quantities become infinite or change abruptly are called singularities. The presence of phase dislocations (singularities) in the wavefront of a light beam determines both the phase and intensity structure around them. As the phase becomes indeterminate at singularities, both the real and the imaginary parts of the field amplitude (that is, also the field intensity) vanish. The characteristic helical phase profiles of optical vortices are described by exp(imθ) multipliers, where θ is the azimuthal coordinate and the integer number m is their topological charge, also called dislocation strength or winding number.Recalling the fact that in free space the Poynting vector gives the momentum flow, for helical phase fronts the Poynting vector has an azimuthal component that produces an orbital angular momentum parallel to the axis of the beam. The momentum circulates around the beam axis, so such beams are said to contain an optical vortex. As has been shown 1-3 , an m-fold charged optical vortex beam carries an orbital angular momentum of mh per photon independent of the spin angular momentum (that is, the polarization state). It was shown that transitions that are forbidden by known selection rules in the electric and magnetic dipole approximation seem to be allowed when using optical vortex beams 10 . This provides a new degree of freedom in the spectroscopy of forbidden transitions. Multi-coloured optical vortices can be generated through a nonlinear frequency-conversion process such as second-harmonic generation 16 or four-wave mixing, which is an important process in the white-light vortex generation. However, as predicted in ref. 11 and observed in ref. 12, vortex breakup in self-focusing nonlinear media is an important issue for supercontinuum vortex genera...

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Strong-field physics with singular light beams

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“…We consider higher harmonic generation (HHG) from an atomic gas exposed to a laser beam [17]. The electrons are bound in orbit around the nucleus, trapped by a Coulomb potential.…”

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confidence: 99%

“…Much work has gone into optimising the harmonic spectrum with respect to a certain target experimentally by varying the incident laser pulse, see e.g. the review [17]. The aim of this paper is to improve the accuracy of simulation of the HHG process, as well as other applications involving electron dynamics.…”

Section: Introductionmentioning

confidence: 99%

Accelerated convergence for Schrödinger equations with non-smooth potentials

Kieri

2014

Bit Numer Math

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When numerically solving the time-dependent Schrödinger equation for the electrons in an atom or molecule, the Coulomb singularity poses a challenge. The solution will have limited regularity, and high-order spatial discretisations, which are much favoured in the chemical physics community, are not performing to their full potential. By exploiting knowledge about the jumps in the derivatives of the solution we construct a correction, and show how this improves the convergence rate of Fourier collocation from second to fourth order. This allows for a substantial reduction in the number of grid points. The new method is applied to the higher harmonic generation from atomic hydrogen.

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