Bulletin of the American Physical Society
40th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 54, Number 7
Tuesday–Saturday, May 19–23, 2009; Charlottesville, Virginia
Session X6: Focus Session: Frontiers in Strong Field Physics at Long Wavelengths |
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Chair: Chii-Dong Lin, Kansas State University Room: Minor Hall 125 |
Saturday, May 23, 2009 10:30AM - 11:00AM |
X6.00001: Strong field atomic physics at mid-infrared wavelengths Invited Speaker: The development of intense, ultrafast laser sources in the mid-infrared (1 $\mu $m $< \quad \lambda \quad <$ 5 $\mu $m) region enables new opportunities in strong-field physics, control of electronic motion and attosecond science. Systematic investigations of the wavelength scaling in this region pave the way to the realization of brighter and shorter attosecond light sources using longer-wavelength driving fields. We will discuss two aspects of mid-infrared laser-atom interaction. First, high harmonic generation leading to intense, ultrashort XUV pulses and attophysics have properties which scale favorably in the mid-infrared. One of them is the group delay dispersion, also known as attochirp, of harmonics generated in gases. It has been identified as the main intrinsic limitation to the duration of Fourier-synthesized attosecond pulses. Theory implies that the attochirp can be decreased at longer wavelength. I will discuss the first measurement of the wavelength dependence of the attochirp and show that a 2 $\mu $m driving wavelength reduces the attochirp with respect to 0.8 $\mu $m at comparable intensities, as predicted . Second, we have revisited strong field ionization of atoms, of which, over the past thirty years, extensive studies have revealed both quantum and classical aspects: the electron wavepacket drift, quiver and rescattering motions lead to a seemingly complete picture of the fundamental laser-atom interaction. The photoelectron energy spectra (Above-threshold Ionization) are thus very well understood. However, with long wavelength (mid-infrared) lasers, an effect which appears to have eluded observation so far is revealed: the photoelectron energy distribution manifests an unexpected characteristic spike-like structure at low energy. This feature, observed in all investigated atoms and molecules, appears universal. Although the structure is qualitatively reproduced by numerical solutions of the time-dependent Schr\"{o}dinger equation, its physical origin is not yet identified. If, of course, the non-relativistic Schr\"{o}dinger equation appears to be numerically correct, the observed feature does not fit in the well established picture which provides a clear classical foundation to strong field atomic ionization. [Preview Abstract] |
Saturday, May 23, 2009 11:00AM - 11:12AM |
X6.00002: Scaling of phase matching of high-order harmonics into the multi-keV x-ray region at low ionization and large density-length products Tenio Popmintchev, Ming-Chang Chen, Alon Bahabad, Michael Gerrity, Paul Arpin, Matthew Seaberg, Richard Sandberg, Margaret Murnane, Henry Kapteyn, Pavel Sidorenko, Oren Cohen, Sterling Backus, Xiaoshi Zhang, Greg Taft, Ivan Christov In this work we investigate theoretically and experimentally how phase-matched high harmonic generation scales to multi-keV x-ray photon energies. We find that by using mid-infrared driving lasers of moderate intensity, this process presents an experimentally feasible and straightforward route for generating bright, fully coherent, beams even at multi-keV photon energies. We experimentally verify our predictions by demonstrating macroscopic phase-matched up-conversion of mid-IR laser light to water-window soft x-rays over extended distances in a high-density medium and moderate ionization levels. Finally, we show that the optimum phase matching pressures, absorption limited medium lengths, and macroscopic high harmonic yields scale very favorably with increasing driving laser wavelengths and x-ray photon energies, compensating for the decreasing single-atom signal strength at long driving wavelengths. [Preview Abstract] |
Saturday, May 23, 2009 11:12AM - 11:24AM |
X6.00003: Temporal Characterization of Below-Threshold Harmonics in a Scaled Keldysh System E. Power, A.M. March, F. Catoire, E. Sistrunk, K. Krushelnick, P. Agostini, L.F. DiMauro We report an experimental and theoretical study of below-threshold high harmonics created by the scaled Keldysh system interaction of a $3.6\mu\textrm{m}$ laser and a cesium target with Keldysh parameter $\gamma\sim 1$; in the Keldysh picture this interaction is the scaled equivalent of an 800nm laser illuminating an argon target. Harmonic orders $5-13$ were temporally characterized using cross-correlation frequency-resolved optical gating (XFROG); this scheme is sensitive to the relative delay between harmonic orders, $\textrm{d}\phi/\textrm{d}q$, and also provides a complete reconstruction of the individual harmonic orders. Through the use of unconventional dispersion management we completely account for the dispersion in our heat pipe exit window, allowing access to $\textrm{d}\phi/\textrm{d}q$ and the spectral phase $\phi(\omega-\omega_q)$ inside the heat pipe. We show that below-threshold harmonics exhibit negative dispersion. Comparison with a time-frequency analysis of harmonic emission times performed on 1-D TDSE simulation results will be presented. The observed $\textrm{d}\phi/\textrm{d}q$, pulse durations for individual harmonic orders, and intensity scaling strongly suggest non-perturbative below-threshold harmonic generation. [Preview Abstract] |
Saturday, May 23, 2009 11:24AM - 11:36AM |
X6.00004: Generation of strong-field below threshold harmonics Jennifer Tate, Mette Gaarde, Kenneth Schafer We study the production of low energy and below threshold harmonics in xenon excited by a short-pulse 1070 nm laser. The frequently used method of calculating harmonic generation at the single atom level via the strong-field approximation is insufficient to describe these harmonics due to the increased importance of the atomic potential. To more accurately model this system we have developed a code that computes the single-atom response by direct numerical integration of the time-dependent Schr\"odinger equation and couples this to a non-adiabatic solution of the Maxwell wave equation. This gives us the ability to calculate the combined microscopic-macroscopic response of the gas system while treating the laser electric field and the atomic potential on an equal footing. Early results show that the below threshold harmonics include a contribution with a strongly intensity-dependent dipole phase, analogous to the familiar long trajectory in above threshold high harmonic generation. [Preview Abstract] |
Saturday, May 23, 2009 11:36AM - 12:06PM |
X6.00005: Strong field Ionization in the Far Infrared Limit Invited Speaker: In the last few years, strong-field ionization has drawn considerable interest since it is the main response of atoms and molecules to a strong laser field and because of its importance in processes like high-harmonic generation. Ionization mechanisms are usually classified in terms of one of two physical processes, namely multi-photon ionization or field ionization. The Keldysh parameter $\gamma=(I_{P}/2U_{P})^{2}$, with $U_{P}$ the ponderomotive energy, allows to distinguish between these types of ionization. In the multi-photon regime ($\gamma>1$) the ionization yield scales with $S\sim I^{\alpha}$, with $I$ the laser intensity and $\alpha$ the minimum number of photons needed to cross the ionization threshold. In the field ionization limit ($\gamma<1$), ionization takes place by passing over or tunnelling through the barrier that results from combining the Coulomb potential and the laser electric field. Due to technical limitations, most of experiments on strong-field ionization of atoms were performed in a relatively narrow wavelength range from the ultra-violet to the mid-infrared regime. The development of light sources like the Free Electron Laser for Intracavity Experiments (FELICE) at Rijnhuizen allows strong-field ionization experiments to be performed at previously inaccessible far-infrared wavelengths. Here, we report an experiment on strong-field ionization in Xenon and Argon using this new laser source. Starting from high-lying Rydberg states (n$>7$), complex photoelectron velocity distributions with resolved ATI structure characteristic of the multi-photon regime are observed which have been studied both as a function of the prepared atomic state and the wavelength of the incoming radiation. For low-lying Rydberg states (n$<7$), the observed ATI structure disappears, meaning that ionization takes place in the field ionization regime, involving the absorption of several hundred photons. [Preview Abstract] |
Saturday, May 23, 2009 12:06PM - 12:18PM |
X6.00006: Multiphoton Assisted Recombination: Beyond the Simpleman's Model Edward Shuman, Tom Gallagher, Bob Jones We have observed multiphoton assisted recombination in the presence of a 38.8 GHz microwave field. Stimulated emission of up to ten microwave photons results in energy transfer from continuum electrons, enabling recombination. The maximum electron energy loss is far greater than the 2 $U_p$ predicted by the standard ``simpleman's'' model. The data are well reproduced by both an approximate analytic expression and numerical simulations in which the combined Coulomb and radiation fields are taken into account. [Preview Abstract] |
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