Summary form only given. Fundamental processes in atoms, molecules, as well as condensed matter are triggered or mediated by the motion of electrons inside or between atoms. Electronic dynamics on atomic length scales tends to unfold within tens to thousands of attoseconds (1 attosecond [as] = 10-18 s). Recent breakthroughs in laser science are now opening the door to watching and controlling these hitherto inaccessible microscopic dynamics. The key to accessing the attosecond time domain is the control of the electric field of (visible) light, which varies its strength and direction within less than a femtosecond (1 femtosecond = 1000 attoseconds). Atoms exposed to a few oscillations cycles of intense laser light are able to emit a single extreme ultraviolet (XUV) burst lasting less than one femtosecond. Full control of the evolution of the electromagnetic field in laser pulses comprising a few wave cycles have recently allowed the reproducible generation and measurement of isolated sub-femtosecond XUV pulses, demonstrating the control of microscopic processes (electron motion and photon emission) on an attosecond time scale. These tools have enabled us to visualize the oscillating electric field of visible light with an attosecond "oscilloscope", to control single-electron and probe multi-electron dynamics in atoms, molecules and solids. Recent experiments hold promise for the development of an attosecond X-ray source, which may pave the way towards 4D electron imaging with sub-atomic resolution in space and time.

Attosecond Physics

High-harmonic generation (HHG) traditionally combines ~100 near-infrared laser photons to generate bright, phase-matched, extreme ultraviolet beams when the emission from many atoms adds constructively. Here, we show that by guiding a mid-infrared femtosecond laser in a high-pressure gas, ultrahigh harmonics can be generated, up to orders greater than 5000, that emerge as a bright supercontinuum that spans the entire electromagnetic spectrum from the ultraviolet to more than 1.6 kilo-electron volts, allowing, in principle, the generation of pulses as short as 2.5 attoseconds. The multiatmosphere gas pressures required for bright, phase-matched emission also support laser beam self-confinement, further enhancing the x-ray yield. Finally, the x-ray beam exhibits high spatial coherence, even though at high gas density the recolliding electrons responsible for HHG encounter other atoms during the emission process.

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Bright Coherent Ultrahigh Harmonics in the keV X-ray Regime from Mid-Infrared Femtosecond Lasers

https://www.physik.uni-augsburg.de/theo1/hanggi/Papers/213.pdf

Driven quantum tunneling

Atoms exposed to intense light lose one or more electrons and become ions. In strong fields, the process is predicted to occur via tunnelling through the binding potential that is suppressed by the light field near the peaks of its oscillations. Here we report the real-time observation of this most elementary step in strong-field interactions: light-induced electron tunnelling. The process is found to deplete atomic bound states in sharp steps lasting several hundred attoseconds. This suggests a new technique, attosecond tunnelling, for probing short-lived, transient states of atoms or molecules with high temporal resolution. The utility of attosecond tunnelling is demonstrated by capturing multi-electron excitation (shake-up) and relaxation (cascaded Auger decay) processes with subfemtosecond resolution.

Attosecond real-time observation of electron tunnelling in atoms

Above-threshold ionization: From classical features to quantum effects

/pdf/dynamic-hyperpolarizability-and-two-photon-detachment-in-the-29yhydnyo9.pdf

Dynamic hyperpolarizability and two-photon detachment in the presence of a strong static electric field: Application to H -

On the basis of the generalized Sturm expansion of the radial part of the Coulomb Green function, a computational method is proposed and numerical results are presented for the dynamic hyperpolarizability γ and the corrections E
(4) (quadratic in the light intensity) to the quasi-energy of the ground and excited states of hydrogen with principal quantum numbers n≤5 in a monochromatic light field. In this approach, the problem is reduced to the summation of well-convergent double series of the hypergeometric kind, which ensures reliable numerical results both for states with a large n, and in a wide range of field frequencies ω, including the above-threshold frequency range of ℏω≪|E

 n
 | (|E

 n
 | is the ionization potential of the state |nlm〉 under investigation). We consider the frequency dependence of γ and E
(4), their differences for the cases of linear and circular polarizations of the field, and the relation between their real and imaginary parts, which determine the laser field-induced corrections to the position and width of energy levels. For n=5, the significant role of mixing the |nlm〉 states with different values of l by a laser field in the region of resonances on intermediate bound states is demonstrated. The linear (in intensity) corrections to the photoionization cross section for excited states are analyzed and the threshold intensity corresponding to the onset of atomic level stabilization is estimated for a number of states with n=3 and n=5.

Dynamic hyperpolarizabilities of excited states of hydrogen

Vacuum polarization by a strong coulomb field and its contribution to the spectra of multiply-charged ions

A. V. Flegel,1,2 M. V. Frolov,3 N. L. Manakov,3 Anthony F. Starace,1 and A. N. Zheltukhin1,3 1Department of Physics and Astronomy, The University of Nebraska, Lincoln, Nebraska 68588-0299, USA 2Department of Computer Science, Voronezh State University, Voronezh 394006, Russia 3Department of Physics, Voronezh State University, Voronezh 394006, Russia (Received 20 November 2012; published 20 March 2013)

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Control of atomic dynamics in laser-assisted electron-atom scattering through the driving-laser ellipticity

Interference between dipole and quadrupole transition amplitudes in photo-double-ionization of He by an elliptically polarized vuv photon is shown to induce circular dichroism in the case of equal energy sharing. The magnitude of this retardation-induced dichroic effect is estimated and its impact on the nondipole asymmetries of the triply differential cross section is demonstrated.

/pdf/circular-dichroism-at-equal-energy-sharing-in-photo-double-4d6l8o4cgg.pdf

N. L. Manakov

Papers

Dynamic hyperpolarizability and two-photon detachment in the presence of a strong static electric field: Application to H -

Dynamic hyperpolarizabilities of excited states of hydrogen

Vacuum polarization by a strong coulomb field and its contribution to the spectra of multiply-charged ions

Control of atomic dynamics in laser-assisted electron-atom scattering through the driving-laser ellipticity

Circular dichroism at equal energy sharing in photo-double-ionization of He