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The generation of short, intense, mid-infrared laser pulses allows for the exploration of atom–laser interactions deep in the tunnelling regime as well as providing the ability to explore scaled interactions. In this paper we present recent experimental and theoretical results for this largely unexplored parameter space.

Laser induced periodic surface structures (LIPSS) were generated via interaction of multiple 90 femtosecond 1900 - 3600 nm mid IR laser pulses (3 -10,000) on single crystal Ge targets. For specific laser parameters, both low and high frequency LIPSS are found together, which are oriented perpendicular to each other. Study of polarization dependence of LIPSS revealed that orientation and symmetry of interaction could be controlled by rotating polarization of laser pulses. Low frequency LIPSS formation was consistent with surface plasmon coupling of laser pulses with excited Ge.

▪ Abstract The unprecedented high intensities made accessible by the development of ultrashort pulse laser systems have uncovered a host of new phenomena and changed the way that we think about atomic and molecular dynamics. We review these developments in the context of some of the basic models that have evolved to explain them. The successes and limitations of simple semiclassical models of strong-field atomic phenomena are discussed. Photodissociation dynamics have shown a rich complexity in strong fields—bond softening, above-threshold dissociation, coulomb explosion—which we review. The promise of the new laser systems in the field of coherent control of molecular dynamics is also discussed.

Over the last fifteen years, 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 [1], 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 μm, or in the language of Keldysh to the multiphoton or mixed ionization regime. It is now technically possible to perform more thorough test, and exploit the scaling laws at wavelengths greater than 1 μm. 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.

Explosions of large Xe clusters (N~ 11,000) irradiated by femtosecond pulses of 850 eV x-ray photons focused to an intensity of up to 10(17) W/cm(2) from the Linac Coherent Light Source were investigated experimentally. Measurements of ion charge-state distributions and energy spectra exhibit strong evidence for the formation of a Xe nanoplasma in the intense x-ray pulse. This x-ray produced Xe nanoplasma is accompanied by a three-body recombination and hydrodynamic expansion. These experimental results appear to be consistent with a model in which a spherically exploding nanoplasma is formed inside the Xe cluster and where the plasma temperature is determined by photoionization heating.

We present results of non-perturbative high-order harmonic generation and photon-assisted tunneling driven by intense mid-infrared laser in solids. These strong-field effects are intimately related and occur in a limit where the laser-field competes with crystal bonding.

Nonlinear absorption of femtosecond kilovolt x-rays in Neon is investigated. A possible two-photon Ionization is observed with the presence of Ne9+ at photon energies below the Ne8+ 1s2 ionization threshold.

(35 Word Limit) High harmonic generation and strong-field ionization provides the methods to generate attosecond extreme ultraviolet pulses. Our research has explored, in particular, how high harmonic generation and strong-field ionization scales with the wavelength of the driving laser. In general, with increasing wavelength, higher frequency harmonic combs can be generated with more favorable intrinsic spectral phase profiles.

A short pulse, high energy, high repetition rate dye amplifier with superior conversion efficiency is reported. The dye amplifier is composed of three single-pass dye cells, longitudinally pumped by a frequency doubled 1 kHz Nd: YLF regenerative amplifier. The dye amplifier yields 3.5 ps, 150 μJ pulses at 595 nm, corresponding to a 12% conversion efficiency. The ASE is 1% or less, and the transverse mode quality is near-Gaussian.

We use the unprecedented LCLS peak power to study nonlinear x-ray multiphoton physics in molecules. We report on fundamental questions concerning the creation and decay of double-core-hole vacancies in N2 by short x-ray pulses.

We observe a rich angular spectrum resulting from the Auger decay of a single K-shell vacancy but an apparent angle independence for the decay of a double core vacancy “localized” to a single atom.

The paper reviews recent advances in high harmonic generation from mid-infrared lasers. The results emphasize the advantages of long wavelength drivers for cut-off extension, GDD reduction, attosecond pulses and reports on below-threshold and liquid harmonics.

We demonstrate filamentation at 2 μm using carrier-envelope phase (CEP) stabilized 55 fs, 330 μJ pulses from an OPA. The ultra-broadband output is self-compressed below 3-optical cycles with 270 μJ and preserves the CEP offset.

This document reports recent theoretical and experimental investigations of strong field ionization and high harmonic generation from mid-infrared lasers at 2 and 4 microns. Numerical solution of the time-dependent Schrodinger equation as well as Strong Field approximation calculations are reported. Photoelectron and high harmonic spectra are discussed. Preliminary experimental results are compared to the theoretical predictions.

The genesis of attosecond pulses signifies a new frontier in time-domain science providing light bursts equivalent to the electronic time-scale. This tutorial will examine the fundamental principles, recent advances and challenges from these sources.

High precision photoelectron energy and angular distributions in helium and neon atoms for a broad intensity range reflect the change in the continuum dynamics that occurs as the ionization process evolves into the pure tunneling regime. Elastic rescattering of the laser-driven free electron from its parent ion core leaves a distinct signature on the spectra, providing a direct quantitative test of the various theories of strong field multiphoton ionization. We show that it takes a relatively complete semi-classical rescattering model to accurately reproduce the observed photoelectron distributions. However, the calculated inelastic rescattering rate fails to reproduce the measured nonsequential double ionization yields.

High-order harmonic generation by means of high-energy, femtosecond laser pulse has proved to be a convenient way to generating coherent radiation in the extreme-ultraviolet (XUV) spectral regions. Recent experiments [1,2] show that harmonic duration is much shorter than the pump one while divergence measurements show that the harmonic beam should be focussable to a fairly small spotsize and reach intensities high enough for nonlinear interactions. In this contribution we report the first electron energy spectrum(*) resulting from non-resonant multiphoton transitions induced in the XUV domain using harmonics produced in xenon by the 120 fs, 10 mJ Ti:sapphire laser at the Palaiseau facility. An annular beam geometry[3] is used to separate the fundamental from the harmonics which are focussed in a magnetic bottle electron spectrometer filled with 10-2 torr of helium by a (W-Re)-coated spherical mirror (R=70mm). The intensity of the pump laser is adjusted to .9 mJ for a cutoff at harmonics 15. Consequently, no one-photon ionization of helium is observed. Photoelectrons are detected only when the XUV radiation reflected by the mirror is focussed in the sensitivity zone of the magnetic bottle (Fig.1). This behavior is the signature of a strong intensity dependence of the ionization process and rules out one-photon processes.

We have developed a high intensity, ultra-short kilohertz laser system operating in the mid infrared, between 3 and 4.5 μm. The output of an ultrashort pulse oscillator (sync-pumped dye laser or mode-locked Ti:Sapphire laser), which is tunable about 800 nm, is amplified in a Ti:Sapphire regenerative amplifier, using chirped pulse amplification. Another pulse at λ=1053 nm, synchronized with the tunable pulse, is amplified in a cw pumped Nd:YLF regenerative amplifier. Mixing the two pulses in a nonlinear crystal, we obtain the difference frequency in the mid-infrared. A schematic of one configuration is shown in the figure below. The 800 nm oscillator in this case is a dye laser synchronously pumped by a mode-locked Nd:YLF laser, which also acts as the seed source for the 1053 nm light, thus providing optical synchronization between the two pulses at the mixing crystal. This configuration yields 1 psec pulses in the mid-infrared with an energy of 70 microjoules, which may be focussed, using f/4 optics, to peak intensities above 1012 W/cm2. The 1053 nm seed source may also be electronically synchronized with a mode-locked Ti:Sapphire oscillator in place of the dye laser, reducing the pulsewidth and increasing the peak intensity by an order of magnitude.

X-ray free-electron lasers (FELs) produce femtosecond x-ray pulses with unprecedented intensities that are uniquely suited for studying many phenomena in atomic, molecular, and optical (AMO) physics. A compilation of the current developments at the Linac Coherent Light Source (LCLS) and future plans for the LCLS-II and Next Generation Light Source (NGLS) are outlined. The AMO instrumentation at LCLS and its performance parameters are summarized. A few selected experiments representing the rapidly developing field of ultra-fast and peak intensity x-ray AMO sciences are discussed. These examples include fundamental aspects of intense x-ray interaction with atoms, nonlinear atomic physics in the x-ray regime, double core-hole spectroscopy, quantum control experiments with FELs and ultra-fast x-ray induced dynamics in clusters. These experiments illustrate the fundamental aspects of the interaction of intense short pulses of x-rays with atoms, molecules and clusters that are probed by electron and ion spectroscopies as well as ultra-fast x-ray scattering.

Over the last few years, strong-field physics has advanced to a state that quantitative tests of theoretical models are now possible experimentally[1-3]. This is particularly important to identify the universality of all strong-field effects, such as above-threshold ionization (ATI), high optical harmonic generation (OHG), and multiple ionization. In this talk, we will report on the progress of high sensitivity electron and ion measurements in the high field limit on inert gas atoms and helium. The experiments are performed with kilohertz laser systems operating at 1 μm and 0.78 μm with 50 ps and 0.1 ps pulse durations, respectively and extend from 1012–16 W/cm2. Thus, the studies are capable of examining the full dynamic range of field interactions from multiphoton to tunnel ionization. Critical comparison to both time-dependent quantum mechanical calculations using a single electron approximation (SAE) and quasi-classical models provide interesting insights into the underlying physics.

We report on our progress in detecting two-electron ionization using electron-electron and electron-ion coincidence measurements. The coincidence techniques have been applied to the multiphoton ionization (MPI) of xenon atoms with 0.527 µm excitation. The results show that direct two electron ionization is not occurring which is in variance with an earlier report. We also present a polarization study on the MPI of helium at 0.62 pm and discuss these results in context of existing models.

Laser induced fluorescence and dispersed emission spectra of C6F6Ar n (n = 1–10) clusters have been studied in a supersonic freejet expansion. By controlling the expansion conditions, we can approximately mass select the clusters and selectively pump the C 6 F 6 + Ar n from the ground electronic state to different vibrational modes of the excited electronic state. We observe that the wavelength resolved emission consists of a broad (about 1300 cm-1, FWHM), Stokes shifted spectrum that is independent of vibrational excitation and indicative of condensed phase emission. This is in contrast to the LIF spectrum, which yields much narrower linewidths. The emission and excitation spectra show an increasing red shift with larger cluster size that saturates at high pressure. A model is proposed for explaining all the above phenomena. A similar experiment is also being performed with C6F6He n clusters.

The Linac Coherent Light Source (LCLS) Project will be an x-ray free-electron laser. It is intended to produce pulses of 800-8,000 eV photons. Each pulse, produced with a repetition frequency of up to 120 Hz, will provide >1012 photons within a duration of less than 200 femtoseconds. The project employs the last kilometer of the SLAC linac to provide a low-emittance electron beam in the energy range 4-14 GeV to a single undulator. Two experiment halls, located 100 m and 350 m from the undulator exit, will house six experiment stations for research in atomic/molecular physics, pump-probe dynamics of materials and chemical processes, x-ray imaging of clusters and complex molecules, and plasma physics. Engineering design activities began in 2003, and the project is to be completed in the middle of 2010. The project design permits straightforward expansion of the LCLS to multiple undulators.

The vinoxy, CH2CHO, free radical has been produced in a supersonic free jet expansion by ArF excimer laser photolysis of a suitable precursor like ethyl vinyl ether. Electronic spectra involving the B 2A″ ↔ X 2A″ transition have been observed at vinoxy temperatures between 2–30 K. Rotational structure as well as spin splittings have been resolved. Analysis of the spectrum yields the electronic term value, vibrational frequencies, rotational and spin‐splitting constants for both electronic states. These molecular parameters provide considerable information about the electronic and geometric structure of the radical.

We report on our investigation of THz emission from femtosecond filaments in argon and propose a single-shot scheme for determination of the absolute carrier envelope phase with a 2-pi ambiguity.

Cluster explosion in ultrashort intense X-ray laser fields have been studied in first experiments at the LCLS with time-of flight techniques. Ion charge states and kinetic energy spectra indicate hydrodynamic and Coulombic plasma expansion contributions.

Explosions of methane clusters driven by intense XFEL pulses were studied during the first experiments at LCLS. Ion fragment distribution dynamics depends strongly on wavelength and pulse width.

Interaction of ultrashort — X-rays laser with xenon doped methane clusters have been studied in the first experiments at LCLS with time-of-flight technique. Xe doping affects the explosion of the CH 4 clusters.

Deploying the so-called ‘Streaking Spectroscopy’ technique at LCLS, we demonstrate a non-invasive scheme for temporal characterization of X-ray pulses with sub-femtosecond resolution. Analyzing the substructure indicates pulse durations on the order of hundreds of attoseconds.