Session N5: Attosecond and Femtosecond Dynamics

Harmonic Generation at Nonadiabatic Crossings of Electronic States

Nonadiabatic crossings of molecular electronic states create coherent superpositions of electronic wave functions.The time evolution of such coherent electronic states on ``attosecond'' time scale depends on the energy separation of these states as a function of internuclear distance [1]. High order harmonic generation, HHG, induced by ultrashort intense laser pulses can be used to monitor such attosecond evolution in dissociating systems such as H3++ to H+ and H2+ [2]. We examine by detailed solutions of the time-dependent Schroedinger equation, TDSE, the HHG spectrum for the dissociating linear H3++ system subjected to an intense few cycle laser pulse at different wave lengths and internuclear distances where an adiabatic avoided crossing occurs between selected electronic states. Criteria for possibly extracting electron-nuclear nonadiabatic couplings from HHG will be presented. \newline [1] G L Yudin, A D Bandrauk, P B Corkum, Phys Rev Lett 96,063002(2006); \newline [2] A D Bandrauk, S Barmaki, G Lagmago Kamta, Phys Rev Lett 98,013001(2007).   

Propagation Effects on Attosecond Pulse Generation in Molecular Media

We investigate numerically on large scale parallel supercomputer the dynamics of ``attosecond'' pulse generation created from high order harmonic generation, HHG, in molecular media. Propagation of an intense 5-fs Ti:Sa (800 nm) in a H2+ gas medium is studied from numerical solution of the coupled time-dependent Schroedinger (TDSE)and Maxwell equations. Exact 1-D and 3-D solutions of the H2+ TDSE are compared as the 1-D molecular model coupled to 3-D Maxwell equations allows for including up to 10 000 molecules on an appropriate grid.The simulations allow for exact calculation of electron ionization and recombination leading to HHG including propagation effects.In particular we have designed mathematical boundary conditions which allow free ionized electrons to be transmitted from one molecule to another. Instabilities such as filamentation of the incident intense (800 nm)pulse are observed as a function of medium density.The effect of 3- photon resonances which occur in atmospheric filamentation are examined using the present novel algorithm.   

Effect of Carrier Envelope Phase On Attosecond Two-Slit Interference

High order harmonics generated by polarization gating have been experimentally demonstrated to be sensitive to the carrier envelope phase. Supercontinuum extreme ultra violet spectra generated by the polarization gating method were mapped for several minutes while changing the thickness of a fused silica plate on the beam path while the CE phase the amplifier and the oscillator were locked. A shift in the XUV spectra associated with appearance of suppercontinuum at pi radian spacing was observed when the gate width was made narrower than one optical cycle. The supercontinuum can be interpreted as a result of emission of a single electron and ion re-collision. When the polarization gate width was wider, only the XUV shift showed up with respect to the change in the CE phase in absence of continuum spectra as in this case, only the emission of many electron-ion re-collisions persist. In either case the XUV spectra observed to be shifted toward increasing photon energy when relative CE phases increases. Observed changes in the orientation of the CE phase affected XUV spectra when the gas cell was moved along the confocal distance can be accounted for the effect of the Guoy phase shift.   

Simulation of carrier-envelope phase effects on polarization gated attosecond spectra

Polarization gating of harmonic generation by a laser field with a time dependent ellipticity has been demonstrated as a powerful method to produce single attosecond pulses. A single attosecond in time domain correspond to an extreme ultraviolet (XUV) supercontinuum in the spectrum domain. Previous numerical simulations and experiments show that the attosecond pulses and the corresponding spectra are sensitive to the variation of the carrier-envelope (CE) phase of the laser pulses. In this work, the mechanism of the effects of CE phase on the polarization gating process is investigated. In the simulation, the laser pulses for polarizating gating are obtained by transforming linearly polarized few-cycle pulses with a quartz plate and an achromatic waveplate. The laser pulse can be decomposed into two orthogonally polarized pulses, i.e., a driving pulses and a gating pulse. The CE phase is defined as the phase of the driving field at the time when the ellipticity of combined pulses is zero. We found that when the CE phase changes from zero to Pi, the XUV spectra evolves from a supercontinuum to discrete harmonic peaks and than back to a supercontinuum. The evolution can be understood by the interference of the two attosecond pulses. The amplitudes and spectral phases of two pulses are controlled by the CE phase and the envelope of the driving field.   

Attosecond Time-Scale Multi-Electron Collisions: Imprints on Double and Single Energy Differential Cross Sections

A finding of our work on triple photoionization of Li in a quasiclassical formulation is a classification scheme which organizes the triply photo-ionizing trajectories in groups according to the respective sequence of electron-electron collisions [ref1]. The collision sequences in triple ionization of Li take place on an attosecond time scale. These electron-electron collision sequences manifest themselves on the level of ensemble averages. The two main collision sequences the three electrons follow to ionize have unique traces in the classical probability densities [ref2]. We have very recently formulated quasiclassically the double energy differential cross sections [ref3]. For small energies we discuss the structure of the double and single energy differential cross sections and show how this structure bares the imprint of the ``T-shaped'' pattern of the three escaping electrons and thus of the electron-electron collision sequences. [ref1] A. Emmanouilidou and J.M. Rost, J. Phys. B 39, 4037 (2006). [ref2] A. Emmanouilidou and J. M. Rost, accepted Phys. Rev. A (2007). [ref3] A. Emmanouilidou, xxx.lanl.gov/physics/0701314.   

AMO Science with the LCLS x---ray FEL

The Linac Coherent Light Source (LCLS), an x-ray free electron laser (xFEL), is currently under construction at the Stanford Linear Accelerator Center. Scheduled for completion in early 2009, the LCLS will produce the world's brightest beams of x-rays over 1.5 -- 15{\AA} in $\sim $200fs pulses at 120Hz. An experimental system to study the interaction of these intense x-ray pulses with atoms, molecules and clusters has been designed and is being built as a part of the project. A Kirkpatrick-Baez mirror system will be used to focus the x-ray beam to a small spot where it will photoionize atoms or molecules introduced from a skimmed pulsed supersonic nozzle. Five time-of-flight electron spectrometers will be arrayed around the interaction region to measure the energy and angular distributions of ejected electrons. One of three different ion spectrometers will be used to measure the production of charge states and/or momenta of the ions produced. A description of instrument and a brief description of the facility will be presented.   

Sweeping the carrier-envelope phase of grating based chirped pulse amplifiers

As a well-developed technique, grating based chirped pulse amplification (CPA) can generate high power laser pulses with durations longer than 10 fs, which can be further compressed to $\sim $5 fs. For such intense, few-cycle pulses, it is crucial to control their carrier-envelope (CE) phase for strong field atomic physics studies. Conventionally, the CE phase of few-cycle pulse was varied by a wedge pair while the CE phase of the pulses from the CPA was stabilized. In this work, we focus on studying the effects of the stability of the grating separation in the stretcher and compressor on the CE phase variation. By feedback controlling the effective distance between two gratings in the stretcher, the relative CE phase of the amplified pulses was stabilized with a 180 mrad rms error. Furthermore, by smoothly changing the locking reference, the relative CE phase was swept through a 2$\pi $ range. The sweeping method was confirmed by the XUV spectrum generated by polarization gating. The correlation between the relative CE phase and the high harmonic peak energy showed a $\pi $ period, which agreed very well with theory.   

Quasi Phase Matching and In-situ Probing of High Harmonic Generation in a Hollow Waveguide Using Counterpropagating Light

We use counterpropagating light to directly observe, in-situ, the coherent buildup of high-order harmonic generation in a hollow waveguide. In this technique, the interfering beam scrambles the quantum phase of the harmonic field, thus suppressing emission from the intersecting region. We measure the phase mismatch for photon energies ($\sim $70 eV in Argon) that cannot be phase matched using conventional approaches. This information allows us to design a pulse train that implements all-optical quasi phase matching in this regime, demonstrating for the first time the use of counterpropagating laser pulses to implement quasi phase matched enhancement of high-harmonic conversion. This technique can be extended to phase match conversion even to very high photon energies.   

Carrier-envelope phase drift of 1mJ, few-cycle pulse produced by hollow fiber

We investigated carrier-envelope (CE) phase drift due to self-phase modulation through a Ne filled hollow fiber. The high energy seed pulses in our experiments were generated using a grating-based chirped pulse amplification laser system. The system produces 25 fs pulses with output energy of 2.5 mJ at 1 kHz. The CE phase of the amplifier is stabilized by feedback controlling the grating spacing. The output beam was focused into a hollow fiber with interaction length of $\sim $1 m. To accommodate the seed energy, large core diameter hollow fibers filled with Ne gas were used for spectral broadening. The measured output pulse duration is 5.6 fs with energy of 1.2 mJ after dispersion compensation by chirp mirrors. The CE phase stability was monitored by out-loop $f$-to-2$f$ interferometery after the fiber. The dependence of the CE phase drift of a few-cycle pulse on the stability of the input laser and the gas pressure was investigated as well. Furthermore, with seed pulse power locked, the CE phase of the pulse is controlled to a standard deviation of 370 mrad. The peak power of the CE phase stabilized pulses, 0.2 TW, is twice that previously generated. The significance of seed pulse energy stability for CE phase stabilization of few-cycle laser pulses is demonstrated.   

Laser induced ultrafast electron emission from a field emission tip

We show that by focusing the output of a femtosecond oscillator on a tungsten nanometer field emission tip we can create electron pulses that are less than 100 fs in duration. There are different possible mechanisms for the observed electron emission: thermal excitation, multi-photon absorption and field emission. The process dominating the electron emission will ultimately determine the electron pulse duration. For different DC offset voltages we find the intensity dependence of the emission process is proportional to the light intensity to the 2nd, 3rd or 4th power. This indicates that multi-photon absorption is the dominant process. Our findings are in stark contrast to those presented by Hommelhoff \textit{et al}. [1,2]. They claim that the electron emission process is dominated by optical field emission, which seems to be inconsistent with the Keldysh parameter ($\gamma >$1), and our observed electron emission intensity dependence. Regardless of the ultimate pulse duration of the emitted electron packets this source may be important for UEM and other fundamental electron physics studies. [1] Peter Hommelhoff, et al. Phys. Rev. Lett. \textbf{96}, 077401 (2006). [2] Peter Hommelhoff, et al. Phys. Rev. Lett. \textbf{97}, 247402 (2006).   

The Laser-Assisted Photoemission from Surfaces

The Laser-assisted photoelectric effect (LAPE) is a powerful tool for characterizing femtosecond-to-attosecond EUV pulses, and for time-resolved spectroscopy of electron dynamics in atoms. Recently, we observed this process for the first time in the original manifestation of the photoelectric effect i.e. photoemission from surfaces. Irradiating a surface in infrared light as an EUV photon ejects an electron from a surface, this electron can also absorb or give-up energy from the infrared field. We can extract sideband amplitudes from the continuous photoemission spectra, making it possible to record a cross-correlation between the two beams. This result is of interest because LAPE has the potential to study ultrafast, femtosecond-to-attosecond time-scale electron dynamics in solids and in surface-adsorbate systems where complex, correlated, electron relaxation processes are expected. However to extend these applications of LAPE to surfaces, it must be unambiguously distinguished from hot electron excitation, above-threshold photoemission, and space charge acceleration, as these~effects can potentially lead to similar modifications of the photoemission spectrum. We present new data that reveals surface LAPE in a regime where a wealth of surface-adsorbate dynamics is known to occur.