Bulletin of the American Physical Society
43rd Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 57, Number 5
Monday–Friday, June 4–8, 2012; Orange County, California
Session H4: Focus Session: Strong-Field Light-Matter Interactions |
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Chair: Jun Ye, JILA/University of Colorado Room: Garden 1-2 |
Wednesday, June 6, 2012 10:30AM - 11:00AM |
H4.00001: Strong-field atomic physics in the Classical Limit Invited Speaker: Louis DiMauro Over the last decade, the tailoring of a light field for manipulating the dynamics of a system at the quantum level has taken a prevalent role in modern atomic, molecular and optical physics. As first described by Keldysh, the ionization of an atom by an intense laser field will evolve depending upon the light characteristics and atomic binding energy. Numerous experiments have thoroughly investigated the dependence of the intensity and pulse duration on the ionization dynamics of inert gas atoms. However, exploration of the wavelength dependence has been mainly limited to wavelengths less than 1 $\mu $m, or in the language of Keldysh to the multiphoton or mixed ionization regime. It is now technically possible to perform more thorough test, and perhaps exploit, the scaling laws at wavelengths greater than 1 $\mu $m. In addition, excitation with mid-infrared light augments a variety of atomic systems which will tunnel ionize, as well as posing different model atomic structure, e.g. one- and two-electron like systems. This talk will examine the implication of the strong-field scaling as it pertains to the production of high energy particles and the generation of attosecond pulses. We will interpret the intense laser-atom interaction using a semi-classical trajectory model. [Preview Abstract] |
Wednesday, June 6, 2012 11:00AM - 11:30AM |
H4.00002: High harmonic spectroscopy and time-resolved holography with photoelectrons Invited Speaker: Misha Ivanov I will describe recent applications of high harmonic generation for tracking attosecond dynamics of electrons and holes in molecules, and our hopes to use photo-electron spectra for the same purpose. Interaction of intense infrared laser light with atoms and molecules leads to rich dynamics which presents unique combination of quantum and classical physics, ripe with unusual opportunities for imaging dynamics of electrons and nuclei at the time-scale from about 100 attoseconds to a few femtoseconds. As the infrared laser field strips an electron from an atom or a molecule, the electron starts to oscillate in the laser field. Energy E of these oscillations scales linearly with laser intensity I and quadratically with wavelength $\lambda$ and can easily exceed 100 eV for typical experimental conditions. Re-encounter of the electron with the parent ion during such oscillations leads to several effects, including (i) high harmonic generation, which results from recombination of the returning electron with hole left in the ion, and (ii) electron parent-ion diffraction and electron holography, which results from electron-parent ion scattering. These processes encode spatial and temporal information about the parent ion. Spatial resolution can be better than an angstrom, courtesy of the electron de-Broglie wavelength. Temporal resolution can exceed 100 attoseconds, thanks to the dependence of the returning electron energy on the instant of its return: this energy changes from almost zero to the maximum value in less than half of the laser cycle T (T=2.6 fsec for $\lambda$=800 nm). I will first introduce the basic ideas underlying time-resolved electron holography and show recent proof-of-principle experimental results. The bulk of this talk will focus on high harmonic spectroscopy. The properties of high harmonic radiation - amplitude, phase, and polarization - encode detailed information about attosecond to few-femtosecond motion of electrons and light nuclei in the molecule. Experimental challenge is to completely characterize the emitted radiation, measuring not only light intensity but also phase and polarization. Theoretical challenge is to interpret the experimental data, taking into account highly nonlinear, non-perturbative nature of laser-induced dynamics. I will illustrate the potential of the technique by showing several examples of successful joint experimental and theoretical efforts, which gave us sometimes unexpected insight into core rearrangement during strong-field ionization. I will also show results of using high harmonic generation to time-resolve electron tunnelling from atoms and molecules. [Preview Abstract] |
Wednesday, June 6, 2012 11:30AM - 11:42AM |
H4.00003: High Harmonic Spectroscopy with oriented molecules E. Frumker, N. Kajumba, J.B. Bertrand, H.J. Worner, C.T. Hebeisen, P. Hockett, M. Spanner, S. Patchkovskii, G.G. Paulus, D.M. Villeneuve, P.B. Corkum We report the first measurement of high harmonics from oriented gas samples. We show that attosecond and re-collision science provides a detailed and sensitive probe of molecular asymmetry. On each $1/2$ cycle of an intense light pulse, laser-induced tunnelling extracts an electron wave packet from the molecule. When the electron wave packet recombines, alternately from one side of the molecule or the other, its amplitude and phase asymmetry determines the even and odd harmonics radiation that it generates. We determine the phase asymmetry of the attosecond XUV pulses emitted when an electron recollides from opposite sides of the CO molecule, and the phase asymmetry of the recollision electron just before recombination. [Preview Abstract] |
Wednesday, June 6, 2012 11:42AM - 11:54AM |
H4.00004: High-order Harmonic Generation from Molecular Hydrogen Michael Chini, Xiaowei Wang, Qi Zhang, Kun Zhao, Yi Wu, Zenghu Chang High-order harmonics generated in molecular hydrogen gas by an 800 nm driving laser are compared to those generated in argon gas under the same driving laser intensity and gas pressure. We find that the yield of HHG from hydrogen gas is approximately an order of magnitude less than that of argon HHG although the ionization probabilities of the two gases are equal at intensities above 3$\times $10$^{14}$ W/cm$^{2}$. We propose that the reduced HHG yield results from the low recombination cross-section of hydrogen. [Preview Abstract] |
Wednesday, June 6, 2012 11:54AM - 12:06PM |
H4.00005: High Harmonic Generation in Laser-Assisted Radiative Attachment or Recombination Processes Alexander V. Flegel, Alexander N. Zheltukhin, Mikhail V. Frolov, Nikolai L. Manakov, Anthony F. Starace Resonant enhancements are predicted in cross sections $\sigma_n$ for laser-assisted radiative attachment or electron-ion recombination accompanied by absorption of $n$ laser photons. These enhancements occur for incoming electron energies at which the electron can be attached or recombined by emitting $\mu$ laser photons followed by emission of a spontaneous photon upon absorbing $n+\mu$ laser photons. The close similarity between rescattering plateaus in spectra of resonant attachment/recombination and of high-order harmonic generation is shown based on a general parametrization for $\sigma_n$ and on numerical results for $e-H$ attachment. [Preview Abstract] |
Wednesday, June 6, 2012 12:06PM - 12:18PM |
H4.00006: Quantitative Measurement of Phase Matching Conditions in Higher Order Harmonic Generation Sudipta Mondal, Frederic Condin, Philipp Klaus, Carlos Trallero, Benjamin Wilson, Kristen Gould, Erwin Poliakoff HHG has proven to be a very sensitive probe for the electronic structure of atoms and molecules. Since the generation of the harmonics is a macroscopic process, it depends on phase matching conditions. Macroscopic features of phase matching in high harmonics generation are poorly understood and also very difficult to maintain. In this paper we study phase matching in high harmonic generation quantitatively, so that harmonics can be used as an atomic and molecular probe. We measured HHG spectrum with varying laser intensity and focusing conditions. We also change the phase of the input beam by clipping it with an iris, changing it Gaussian to Bessel beam, and observe two regions of phase matching for Gaussian beam becomes one in case of Bessel beams. [Preview Abstract] |
Wednesday, June 6, 2012 12:18PM - 12:30PM |
H4.00007: Visualizing electron wavepacket dynamics in a strong laser field K. Ellen Keister, Daniel D. Hickstein, Predrag Ranitovic, Paul Arpin, Xibin Zhou, Craig W. Hogle, Bosheng Zhang, Chengyuan Ding, Margaret M. Murnane, Henry Kapteyn, Stefan Witte, Xiao-Min Tong, N. Toshima, Ymkje Huismans, Marc J.J. Vrakking, Per Johnsson Strong-field ionization, combined with 2D electron momentum imaging, has the potential to become a revolutionary tool for probing atomic and molecular structures on the femtosecond timescale. Major features apparent in intense-field photoelectron spectra have been shown to result from electrons scattered by the Coulomb potential that accumulate a different phase and interfere with electron trajectories that do not scatter. However, other features in these photoelectron spectra still remain to be explained. In this work, we use mid-infrared driving lasers to identify new structures in the low-energy photoelectron spectra from atoms, which can be unambiguously attributed to multiple sequential encounters of the laser-driven photoelectrons with the parent ion. This interpretation is obtained using a simple plane-spherical wave model, which provides physical insight into strong-field processes, and quantum-mechanical simulations validate this simple model. Reliably extracting structural information, especially dynamically changing molecules, requires a better understanding of the origin of all the photoelectron spectral features as a function of molecular excitation, orientation, and bond length. [Preview Abstract] |
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