Bulletin of the American Physical Society
2005 36th Meeting of the Division of Atomic, Molecular and Optical Physics
Tuesday–Saturday, May 17–21, 2005; Lincoln, Nebraska
Session B4: Strong Field Atomic and Plasma Processes |
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Chair: Cornelis (Kees) J. Uiterwaal, University of Nebraska-Lincoln Room: Burnham Yates Conference Center Hawthorne |
Wednesday, May 18, 2005 10:30AM - 10:42AM |
B4.00001: Femtosecond Optical Vortex Igor Mariyenko, Chad Petersen, James Strohaber, Cornelis Uiterwaal Given that the rescattering ionization probability is influenced by the polarization of the radiation (which is related to spin angular momentum), we have set out to investigate similar effects of optical Orbital Angular Momentum. It is well-known that Optical Vortices possess Orbital Angular Momentum. In units of $\hbar $ its value per one photon is equal to the topological charge of the vortex, i.e. any integer number [1]. A number of methods to produce optical vortices in monochromatic laser beams are already established (see [2], and refs. therein). Unfortunately, none of these is suitable for broadband laser radiation like femtosecond pulses. We present a method to create Optical Vortices in a femtosecond laser beam. Our setup utilizes a thin phase hologram to produce a doughnut Optical Vortex. Currently, we are extending our technique to make it suitable for use in high-energy femtosecond pulses; at the conference we expect to present results on a high-intensity Optical Vortex. (Separate poster contributions discuss rescattering and our ion detection technique). \newline \par\noindent Work supported in part by NSF. IM acknowledges support through a Jorgensen Postdoctoral Fellowship of UNL. \newline \par\noindent [1] L. Allen \textit{et al.}, \textit{Phys. Rev. A} \textbf{45,} 81 (1992); [2] M. Vasnetsov and K. Staliunas (Ed.), \textit{Optical Vortices} (Nova Science, NY, 1999) [Preview Abstract] |
Wednesday, May 18, 2005 10:42AM - 10:54AM |
B4.00002: Time-dependent studies of photon-induced processes in He S. X. Hu, J. Colgan, L. A. Collins The fully three-dimensional Schr\"odinger equation for two-electron systems, such as He exposed to intense laser pulses, has been numerically solved within the time-dependent close-coupling scheme. The Real-Space Product Formula (RSPF) has been implemented in our calculations in the time propagation and is found to have advantages over other propagators. The resulting cross-sections of two-photon double ionization and the decay rates for autoionization states are in very good agreement with those obtained in experiments and by other methods. In addition, the time-dependent method enables us to examine the detailed dynamics of these processes. [Preview Abstract] |
Wednesday, May 18, 2005 10:54AM - 11:06AM |
B4.00003: COLTRIMS Studies of Correlation in the Sequential Release of Two Electrons from Ar and Ne by Short Laser Pulses Chakra Maharjan, Ali Alnaser, Xiao Tong, Predrag Ranitovic, Shambhu Ghimire, Bing Shan, Zenghu Chang, Igor Litvinyuk, Charles Cocke We have measured momentum distributions of doubly charged argon and neon ions generated by circularly polarized short laser pulses. We measure the full vector Momenta of the ions using cold target recoil ion momentum spectroscopy (COLTRIMS). We use laser pulses centered at 800 nm wavelength, 8 fs pulse duration, 1KHz repetition rate with peak intensities between 0.1 and 2.2X10$^{16}$W/cm$^{2}$. Marked structure in the momentum distributions is seen and interpreted as due to the sequential release of the first and second electrons at different times, and thus with different Momenta, during the rise of the pulse. The ion momentum distributions map the vector sum of the two Momenta, thus provide the fast clock for timing the relatives release times of the two electrons. From the measured spectra the release times of both electrons can be deduced. The results will be compared to model calculations. [Preview Abstract] |
Wednesday, May 18, 2005 11:06AM - 11:18AM |
B4.00004: Multiphoton ionization of xenon in the VUV regime Robin Santra, Chris H. Greene In a recent experiment at the VUV free-electron laser facility at DESY in Hamburg, the generation of multiply charged ions in a gas of atomic xenon was observed. This paper develops a theoretical description of multiphoton ionization of xenon and its ions. The numerical results lend support to the view that the experimental observation may be interpreted in terms of the nonlinear absorption of several VUV photons. The method rests on the Hartree-Fock-Slater independent-particle model. The multiphoton physics is treated within a Floquet scheme. The continuum problem of the photoelectron is solved using a complex absorbing potential. Rate equations for the ionic populations are integrated to take into account the temporal structure of the individual VUV laser pulses. The effect of the spatial profile of the free-electron laser beam on the distribution of xenon charge states is included. An Auger-type many-electron mechanism may play a role in the VUV multiphoton ionization of xenon ions. [Preview Abstract] |
Wednesday, May 18, 2005 11:18AM - 11:30AM |
B4.00005: Effective Range Approach for High Harmonic Generation by Negative Ions M.V. Frolov, A.V. Flegel, N.L. Manakov, Anthony F. Starace We extend a recently-developed model-independent quantum approach for describing laser detachment of an electron with a non-zero angular momentum [1] to the case of high harmonic generation (HGG) by a negative ion. Our results are mainly focused on the dependence of high harmonic yield on the spatial symmetry of the bound state of a weakly-bound electron. Our numerical results for H$^-$ and F$^-$ ions (with $s$ and $p$ outer electrons, respectively) and linear laser polarization show that the HHG spectrum for $s$ and $p$-states are significantly different, whereas the plateau cutoff position is insensitive to the bound state symmetry. For F$^-$, harmonic generation rates for the different magnetic sublevels of the active $p$-electron differ by about 3-5 orders of magnitude. We also discuss the accuracy of Keldysh-like approximations for the HHG amplitude. [1] M.V. Frolov et al., Phys. Rev. Lett. {\bf 91}, 053003 (2003). [Preview Abstract] |
Wednesday, May 18, 2005 11:30AM - 11:42AM |
B4.00006: The Effects of Ionization on the Generation of High Energy 6 fs Pulses Shambhu Ghimire, Bing Shan, Chris Nakamura, Chun Wang, Zenghu Chang So far, the highest energy of the laser pulses with $\sim $6 fs duration generated by the hollow core fiber/chirped mirror compressor technique is below 1 mJ. We studied the effects of multiphoton ionization on the pulse propagation in the fiber that was filled with argon gas. Our experiments showed that when the input pulses are circularly polarized, the output energy can be scaled up by a factor of 1.5 in comparison to that with a linear polarization input. The highest pulse energy obtained with a good spatial mode is 0.6 mJ. A single shot SHG-FROG was used to fully characterize the output pulse which yielded a pulse width of 6.2 fs. The increase of pulse energy is attributed to the lower probability of ionization of the nonlinear medium with a circular polarization input than that with a linear polarization input for the same pulse energy. We believe that the plasma defocusing is a major factor that affects the spatial mode of the beam exiting the fiber. Our results indicate that the defocusing is reduced by using a circularly polarized input. [Preview Abstract] |
Wednesday, May 18, 2005 11:42AM - 11:54AM |
B4.00007: Rescattering Effects in the High-Frequency Regime N.L. Manakov, A.V. Flegel, M.V. Frolov, Anthony F. Starace Characteristic plateau features in the spectra of fundamental strong-field processes (such as high harmonic generation, above-threshold ionization, and laser-assisted electron-atom scattering) are shown to exist for photon energy $E_\gamma$ of the order of the bound electron energy $|E_0|$. The significance of rescattering effects in such a high-frequency (and thus non-tunnelling) regime is supported by accurate quantum analyses of intense Ti-sapphire laser interactions with halide negative ions (in which case $E_\gamma \approx 0.5 |E_0|$). We present a quantum interpretation of rescattering phenomena in the high-frequency regime, in which a multiphoton transition (and not tunneling) is the first step of the rescattering scenario. Our numerical results for the ATD spectrum of F$^-$ support the interpretation of recent experiments [1] in terms of rescattering. [1] J. Pedregosa-Gutierrez et al., Phys. Rev. Lett. {\bf 93}, 223001 (2004). [Preview Abstract] |
Wednesday, May 18, 2005 11:54AM - 12:06PM |
B4.00008: Circularly Polarized Laser Field-Induced Rescattering Plateaus in Electron-Atom Scattering Anthony F. Starace, A.V. Flegel, M.V. Frolov, N.L. Manakov Using effective range theory within a quasienergy state approach, we extend our recent analysis of the energy spectrum of electrons scattered from atoms in the presence of an intense, elliptically polarized laser field [1] to the case of atoms supporting negative ions having weakly bound $p$-electrons. Numerical results for $e-H$ and $e-F$ scattering are presented. In both cases there are two (i.e., low- and high-energy) plateau structures in the electron spectrum for any laser polarization, including for the case of circular polarization. The high-energy plateau is shown to originate from rescattering effects, thus providing the first example of the importance of recattering in the case of circular laser polarization. Basic equations of the rescattering scenario are deduced from our {\it ab initio} quantum results for the scattering amplitude in the quasiclassical limit. [1] A.V. Flegel et al., Phys. Lett. A {\bf 334}, 197 (2005); J. Phys. B {\bf 38}, L27 (2005). [Preview Abstract] |
Wednesday, May 18, 2005 12:06PM - 12:18PM |
B4.00009: Laser Conditioning of Wakefield Electron Beams Scott Sepke, Sudeep Banerjee, Rahul Shah, Anthony Valenzuela, Donald Umstadter Recent experiments at the University of Michigan have demonstrated the selective removal of 200 keV electrons by colliding a Maxwellian laser wakefield electron beam with a high intensity ($I \sim 10^{18}$ W/cm$^{2}$) laser pulse. This technique shows great promise in a number of areas of active research as a tool for creating ultrafast electron bunches with tunable energy and low transverse and longitudinal emittance. Using both analytical and numerical models, the mechanism and scale lengths of this process are illustrated and the importance of longitudinal laser fields are shown. Finally, this method is extended to higher energy particles and shorter pulse lengths. This work supported by Sandia National Labs [Preview Abstract] |
Wednesday, May 18, 2005 12:18PM - 12:30PM |
B4.00010: Optical deflection and temporal characterization of an ultra-fast laser-produced electron beam Sudeep Banerjee, Scott Sepke, Anthony Valenzuela, Rahul Shah, Donald Umstadter The interaction of a laser-produced electron beam with an ultra-intense laser pulse in free space is studied. We show that the optical pulse with a$_{0}$=0.5 imparts momentum to the electron beam, causing it to deflect along the laser propagation direction. The observed 3-degree angular deflection is found to be independent of polarization and in good agreement with a theoretical model for the interaction of free electrons with a tightly focused gaussian pulse, but only when longitudinal fields are taken into account. This technique is used to temporally characterize a sub-picosecond laser-wakefield-driven electron bunch. Applications to modifying electron-beam properties (i.e., emittance, duration and energy spread) are also discussed. [Preview Abstract] |
Wednesday, May 18, 2005 12:30PM - 12:42PM |
B4.00011: Quasiclassical results for two-photon Coulomb Bremstrahlung A. A. Krylovetsky, N. L. Manakov, S. I. Marmo Stimulated two-photon Bremstrahlung (2BrS) is investigated at the scattering of quasiclassical electrons ($Ze^2/\hbar v \gg 1$, $v$ is the electron velocity) from a Coulomb center of charge $Ze$ in the presence of a monochromatic light wave of frequency $\omega$. Despite one-photon Bremstrahlung process, the cross section of 2BrS cannot be found within the framework of classical electrodynamics and one has been obtained as a limit $\hbar \to 0$ in exact quantum results. The quasiclassical 2BrS amplitude is expressed in terms of integrals of McDonald functions $K_{2i\xi}$ dependent on the dimensionless ``classical`` frequency $\xi=Ze^2\omega/mv^3$ and the scattering angle $\theta$. Simple analytical results for differential and total cross sections of 2BrS at low ($\xi \ll 1$) and high ($\xi \gg 1$) frequencies are obtained and comparisons with existing approximate results are presented. Photon polarization dependence of the 2BrS cross section is investigated and it is found that the {\it elliptic dichroism} effect vanishes, but the {\it circular dichroism} is kept even in the quasiclassical limit. Quasiclassical results for {\it spontaneous} emission of two BrS photons (having different frequencies) are also discussed. [Preview Abstract] |
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