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We study attosecond XUV transient absorption in dense gases to investigate the interplay between resonant pulse propagation and laser induced phase shifts. We observe lineshape modifications due to laser-imposed phase and temporal reshaping of XUV

We investigate the role of excited states in High-order Harmonic Generation by studying the spectral, spatial and temporal characteristics of the radiation produced near the ionization threshold of argon by few-cycle laser pulses. We show that the population of excited states can lead either to direct XUV emission through Free Induction Decay or to the generation of high-order harmonics through ionization from these states and recombination to the ground state. By using the attosecond lighthouse technique, we demonstrate that the high-harmonic emission from excited states is temporally delayed by a few femtoseconds compared to the usual harmonics, leading to a strong nonadiabatic spectral redshift., Comment: Accepted at Physical Review Letters

We show that attosecond pulse trains are a natural tool to control strong field processes such as high-order harmonic generation. Coherently combining an attosecond pulse train with an IR driving field, we predict and experimentally confirm enhancement and spectral narrowing of the harmonic yield at photon energies around 90 eV. The use of an attosecond pulse train to seed the harmonic generation process replaces tunneling ionization with a single-photon ionization step, therefore permitting the manipulation of the time–frequency properties of high-order harmonic generation already at the single-atom level.

We characterize the temporal structure of high-order harmonic radiation on both the femtosecond and attosecond time scales. The harmonic emission is characterized by mixed-color two-photon ionization with an infrared femtosecond laser using a Mach–Zehnder interferometer where both pump and probe arms travel completely separate paths. In a first experiment, we measure the duration and chirp of individual harmonics. In a second experiment, we resolve, for the first time with this type of setup, the attosecond beating of several harmonics generated under conditions similar to the first experiment. We suggest that the results of both measurements can be combined to determine the full attosecond time structure of the harmonic emission.

The synthesis and characterization of a new class of electron donor-acceptor substituted stilbenes bearing a phosphonate ester as the electron-withdrawing functionality is described. Phosphorylation of aromatic rings was accomplished using Ni-catalyzed Arbuzov reactions, while Pd-catalyzed Heck-type coupling reactions were employed to construct the stilbene derivatives. Through determination of an electro-optic coefficient (r33), it was demonstrated that the phosphonate ester group is effective as an electron-withdrawing group for stilbene-based second-order NLO materials. © 1997 Elsevier Science Ltd.

High harmonic generation (HHG) is used to measure the spectral phase of the recombination dipole matrix element (RDM) in argon over a broad frequency range that includes the 3p Cooper minimum (CM). The measured RDM phase agrees well with predictions based on the scattering phases and amplitudes of the interfering s- and d-channel contributions to the complementary photoionization process. The reconstructed attosecond bursts that underlie the HHG process show that the derivative of the RDM spectral phase, the group delay, does not have a straight-forward interpretation as an emission time, in contrast to the usual attochirp group delay. Instead, the rapid RDM phase variation caused by the CM reshapes the attosecond bursts., 5 pages, 5 figures

The dynamics of strong-field (\ensuremath{\sim}${10}^{14}$ W/${\mathrm{cm}}^{2}$) multiphoton processes in molecules can be investigated using a simple, collinear model for ${\mathrm{H}}_{2}^{+}$. We discuss properties of this model and report preliminary calculations related to recent experimental results. We find that the excitation and ionization dynamics of the molecular ion depend strongly on the laser wavelength, pulse shape, intensity, and particularly on the initial vibrational state of the molecule. \textcopyright{} 1996 The American Physical Society.

International audience; We present angle- and energy-resolved measurements of photoelectrons produced in strongfield ionization of Xe and Ar using a tunable femtosecond laser in the wavelength range between 600 and 800 nm. Systematic analysis of the experimental data that are quantitatively reproduced by numerical solutions of the time-dependent Schrödinger equation with integration over the laser focal volume, demonstrates the dominance of resonance-enhanced ionization. Continuous variation of the laser wavelength allows the identification of a number of consecutive channel-switching effects with a reliable assignment of the intermediate Rydberg states involved. The appearance of the resonant sub-structure in the electron energy spectra is influenced by the presence of a non-resonant contribution. At relatively low laser intensity a coherent addition of resonant and non-resonant ionization processes is observed. Due to the absence of an intensity dependence in the resonance-enhanced ionization, we observe the persistence of Freeman resonances at the transition to the tunnelling regime.

The synthesis and characterization of a new class of electron donor-acceptor substituted stilbenes bearing a phosphonate ester as the electron-withdrawing functionality is described. Phosphorylation of aromatic rings was accomplished using Ni-catalyzed Arbuzov reactions, while Pd-catalyzed Heck-type coupling reactions were employed to construct the stilbene derivatives. Through determination of an electro-optic coefficient (r33), it was demonstrated that the phosphonate ester group is effective as an electron-withdrawing group for stilbene-based second-order NLO materials. © 1997 Elsevier Science Ltd.

International audience; We present angle- and energy-resolved measurements of photoelectrons produced in strongfield ionisation of Xe using a tunable femtosecond laser. An occurrence of highly oscillatory patterns in the angular distribution at low photoelectron kinetic energy is observed that correlates with channel closing/opening over a wide range of laser parameters. The correlation is investigated both experimentally and by means of systematic analysis of numerical solutions of the time-dependent Schrödinger equation (TDSE). Our experimental and numerical results are in quantitative agreement with the semi-classical model introduced by Arbó et al. (Phys. Rev. A 78, 013406 (2008)), which relates the oscillatory patterns to interference between photoelectrons produced during different cycles of the laser pulse in the course of non-resonant ionisation of the atom. We observe that an increase of the laser intensity eventually leads to qualitative invariance of the pattern, defining a limit on the applicability of the semi-classical model.

We systematically study the influence of phase matching on interference minima in high-harmonic spectra. We concentrate on structures in atoms due to interference of different angular-momentum channels during recombination. For this purpose, we use the Cooper minimum (CM) in argon at $~$47 eV as a marker in the harmonic spectrum. We measure two-dimensional harmonic spectra in argon as a function of wavelength and angular divergence. While we identify a clear CM in the spectrum when the target gas jet is placed after the laser focus, we find that the appearance of the CM varies with angular divergence and can even be completely washed out when the gas jet is placed closer to the focus. We also show that the argon CM appears at different wavelengths in harmonic and photoabsorption spectra measured under conditions independent of any wavelength calibration. We model the experiment with a simulation based on coupled solutions of the time-dependent Schr\"odinger equation and the Maxwell wave equation, thereby including both the single-atom response and macroscopic effects of propagation and phase matching. The single-atom calculations confirm that the ground state of argon can be represented by its field-free $p$ symmetry, despite the strong laser field used in high-harmonic generation. Because of this, the CM structure in the harmonic spectrum can be described as the interference of continuum $s$ and $d$ channels, whose relative phase jumps by $\ensuremath{\pi}$ at the CM energy, resulting in a minimum shifted from the photoionization result. We also show that the full calculations reproduce the dependence of the CM on the macroscopic conditions. We calculate simple phase-matching factors as a function of harmonic order and explain our experimental and theoretical observation in terms of the effect of phase matching on the shape of the harmonic spectrum. Our results emphasize that phase matching must be taken into account to fully understand spectral features related to harmonic spectroscopy. Furthermore, we show that in some cases phase matching can be actively used to enhance the visibility of interference minima in high-harmonic spectra.

A train of attosecond pulses, synchronized to an infrared (IR) laser field, is used to create a series of electron wave packets (EWPs) that are below the ionization threshold in .helium. The ionization probability is found to strongly oscillate with the delay between the IR and attosecond fields twice per IR laser cycle. Calculations that reproduce the experimental results demonstrate that this ionization control results from interference between transiently bound EWPs created by different pulses in the train. In this way, we are able to observe, for the first time, attosecond wave-packet interference in a strongly driven atomic system.

We show that we can probe the components of an attosecond bound electron wave packet by mapping the quantum beat signal produced by a synchronized delayed few-cycle infrared pulse into the continuum. In addition, spectrally overlapping peaks that result from one-, two- or three-photon processes from more or less deeply bound states can in principle be interferometrically resolved with high resolution.

We present a first experiment on attosecond pulse train (APT) assisted high-order harmonic generation, where the APT triggers the release of the electron into the continuum, which leads to an enhancement of harmonic yield.

We calculate the linear chirp rates of high-order harmonics in argon, generated by intense, 810 nm laser pulses, and explore the dependence of the chirp rate on harmonic order, driving laser intensity, and pulse duration. By using a time-frequency representation of the harmonic fields we can identify several different linear chirp contributions. to the plateau harmonics. Our results, which are based on numerical integration of the time-dependent Schrodinger equation, are in good agreement with the adiabatic predictions of the strong field approximation for the chirp rates. Extending the theoretical analysis in the recent paper by Mauritsson et al. [Phys. Rev. A 70, 021801(R) (2004)], we also manipulate the chirp rates of the harmonics by adding a chirp to the driving pulse. We show that the chirp rate for harmonic q is given by the sum of the intrinsic chirp rate, which is determined by the new duration and peak intensity of the chirped driving pulse, and q times the external chirp rate.

The degenerate and nondegenerate two-photon absorption (2PA) spectra for a symmetric and an asymmetric fluorene derivative were experimentally measured in order to determine the effect of intermediate state resonance enhancement (ISRE) on the 2PA cross section delta. The ability to tune the individual photon energies in the nondegenerate 2PA (ND-2PA) process afforded a quantitative study of the ISRE without modifying the chemical structure of the investigated chromophores. Both molecules exhibited resonant enhancement of the nonlinearity with the asymmetric compound showing as much as a twentyfold increase in delta. Furthermore, the possibility of achieving over a one order of magnitude enhancement of the nonlinearity reveals the potential benefits of utilizing ND-2PA for certain applications. To model ISRE, we have used correlated quantum-chemical methods together with the perturbative sum-over-states (SOS) expression. We find strong qualitative and quantitative correlation between the experimental and theoretical results. Finally, using a simplified three-level model for the SOS expression, we provide intuitive insight into the process of ISRE for ND-2PA.

Cesium vapor interacting with a tightly focused, intense midinfrared $(3.4\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m})$ source produces high harmonic radiation in the visible∕UV spectral range. The measured yields of harmonic orders 9--17 are found to yield good quantitative agreement with numerical simulations that include both the single-atom and the macroscopic response. The $5\text{th}--9\text{th}$ harmonic orders are generated with sufficient pulse energies $(\ensuremath{\sim}100\phantom{\rule{0.3em}{0ex}}\text{pJ})$ for direct temporal measurements using an autocorrelation method and when correlated with bandwidth measurements are found not to be transform limited. A blueshift connected to a strong time-dependent ionization appears to be the cause of this spectral broadening.

We measure the chirp rate of harmonics 13 to 23 in argon by cross correlation with a 12 femtosecond probe pulse. Under low ionization conditions, we directly measure the negative chirp due to the atomic dipole phase, and show that an additional chirp on the pump pulse is transferred to the qth harmonic as q times the fundamental chirp. Our results are in accord with simulations using the experimentally measured 815 nm pump and probe pulses. The ability to measure and manipulate the harmonic chirp rate is essential for the characterization and optimization of attosecond pulse trains.

We present initial work involving chemical-structure/nonlinear-optical (NLO) property relations for a set of fluorene derivatives. This is achievable using our femtosecond white-light continuum pump-probe nonlinear spectrometer which can rapidly characterized a sample's two-photon absorption spectrum.

The development of a new class of laser systems, capable of producing intense radiation in the mid-infrared (MIR) regime (photon energies between 0.3 and 0.4 eV), opens the possibility of observing multiphoton processes in a new class of systems with lower ionization potentials than those previously studied. Of particular interest are the alkali metal atoms, which are true one-(valence)-electron systems. We present theoretical calculations of above threshold ionization (ATI) and high harmonic generation (HHG) from alkali metal atoms subject to 3-4 μm laser irradiation. The ATI calculations, which use a multiple gauge propagation method, show a striking dependence in the production of high-order photoelectrons on the electron-ion potential. The HHG calculations illustrate the importance of the strong ground-to-first excited state coupling in multiphoton processes in the alkali metals.

We examine strong field atomic physics in a wavelength region (3-4 microns) where very little work has previously been done. The soft photon energy allows the exploration of one-electron atoms with low binding energies (alkali metals). We find that photoionization spectra differ from rare gas studies at shorter wavelengths due to more complex ion core potentials. Harmonic generation is studied, and we find that harmonic bandwidths are consistent with theory and the possibility of compression to pulse widths much shorter than that of the driving pulse. Harmonic yields in the visible and UV are sufficient for a complete study of their amplitude and phase characteristics.

We present a theoretical and experimental study of the production of very high order photoelectrons from alkali metal atoms interacting with intense, midinfrared radiation. The strength of this process shows an unexpectedly strong species dependence. We find that this dependence can be explained via the difference in the cross section for electron-ion scattering from the different atoms. This allows us to directly relate the high energy portion of the photoelectron spectrum to specific features of the electron-ion potential.

Nonlinear optical (NLO) chromophores are essential components in photorefractive materials. Several stilbene derivatives, functionalized with an electron-donating substituent in the 4-position and electron-withdrawing substitutent in the 41-position, are known to display second-order NLO properties. Commonly used electron-donating groups include ether and amino, while electron-withdrawing groups include nitrile, nitro, and sulfone. Many of the NLO chomophores have absorptions that extend well into the visible wavelength range, potentially interfering with charge generation or sensitization. Additionally, crystallization of NLO chromophores in doped polymer systems often occurs and is undesirable. We wish to report the synthesis and characterization of a new class of electron donor-acceptor substituted low molar mass and polymeric stilbenes. The phosphonate ester functionality is employed as the electron-withdrawing moiety, affording excellent solubility. These materials are single component, i.e., the charge transport functionality is covalently incorporated into the NLO chromophore.

Gas clusters irradiated with ultrashort, high intensity laser pulses can exhibit “ionization ignition” which leads to very high charge states that are significantly higher than those for equivalent laser pulses applied to atoms in the gas phase [1,2]. Moreover, such laser generated plasmas from clusters might serve as bright sources of ultrashort duration, hard x-rays [3-5]. Experimental shapes of the x-ray emission spectra are distinctly non-thermal and show evidence of multiple inner shell holes [6] and high electron kinetic energies [7].

A numerical method for solving the time-dependent Schrodinger equation for a one-electron atomic system in an intense, short-pulsed laser field is presented. An effective potential formalism is proposed and tested for representing the excitation of the valence electrons in rare gases. Results for ion production yields, photoelectron distributions and harmonic conversion are presented and compared to recent experimental results.

We propose a method for producing sub-femtosecond VUV pulses by compressing the high order harmonic radiation emitted from a nonlinear medium driven by an intense, ultrafast laser pulse. We present single atom calculations of the harmonic amplitudes and phases as well as simulations of a VUV compressor. Single harmonics from a 27 fs driving pulse can be compressed to less than 2 fs duration, and several adjacent harmonics with similar phase structure can be combined within the same compressor to produce an attosecond pulse.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Atomic clusters illuminated with ultrashort, high intensity lasers pulses might serve as bright sources of ultrashort duration, hard x-rays. Several experiments carried out by other authors indicate high conversion efficiencies of laser light into x-rays1-3. The shape of the spectra are distinctively non-thermal and show evidence of multiple inner shell holes4. Moreover, very high charge states were generated by irradiation of clusters with ultrashort laser pulses5. These charge states were found to be significantly higher than those for equivalent laser pulses applied to atoms in the gas phase.

Recent calculations of ionization rates for H 2 + by an intense λ=228nm laser show a strong dependence of the rate constant on the internuclear distance R. We have performed comparable time-dependent calculations for H 2 + in fields I≤1×10 15W/cm2 for a wide range of fixed R (2–8au) and wavelengths (228–1064 nm). Indeed we obtain excellent agreement with the results of Chelkowski etal 1 at λ=228nm. At large R we find the ionization rate for all wavelengths rapidly approaches that of H(ls). However, at R≈2au and especially for the longer wavelength lasers, such as the λ=769nm Ti: sapphire laser, the large ionization potential (29eV) of the molecule allows it to easily survive intensities up to I=5×10 14W/cm2 for typical short pulses of, say, 150 fsec.

A surprising array of effects have been observed during high-intensity, short-pulse laser excitation of atoms and molecules. To model these measurements, methods have been developed to solve the time-dependent Schrödinger equation for an atom in a time varying, classical electromagnetic field. Studies on many atomic and molecular systems have been performed over a range of intensities from the regime within which perturbative techniques are valid up to field strengths well above an atomic unit (I > 3.51x1016 W/cm2). These calculations have provided predictions for ionization rates, photoelectron energy and angular distributions and photoemission rates. The effects of ac Stark shifted and intensity broadened intermediate states on the emission processes have been investigated. At high frequency and the highest intensities atoms are found to undergo a transition to a state or states which are surprisingly stable with respect to ionization. This had been predicted using a Floquet approach which did not allow for the time variation of the pulse envelope. The time-dependent calculations, however, show that an appreciable fraction of the electronic wave function can survive the rapid rise of an intense pulse and become stabilized with a greatly reduced ionization rate. Here we present a discussion of some of the most recent results.

In this paper, we describe the results of our general long‐term programmatic goal of investigating the strengths and weaknesses of time‐dependent mean‐field theories for collisions. We have made some progress in: (a) obtaining a better formulation of the theory, which has the exact full Schrodinger equation as one limit and permits appropriate classical treatment of heavy particles correctly coupled to the quantally treated electrons; (b) restructuring our numerical treatment to make it fully three‐dimensional, improve accuracy and decrease cycle time, so that larger problems more in keeping with the mean‐field concept can be treated; and (c) incorporating the electrons in the conduction band of a metal into our quantal treatment, making possible the description of collisions of atoms and ions with solids. Numerical results for protons tranversing a thin metallic foil, among other examples, are presented and discussed.

The theoretical description of multiphoton ionization of atoms by high power, short pulse lasers ultimately rests on the ability to integrate the time-dependent Schrodinger equation. Currently, the most advanced codes available for this purpose are atomic scattering programs, which include few of the features intrinsic to the laser-atom problem. By using a mixed radial mesh/angular momentum state representation of the atomic wavefunctions we find that we can reproduce the results from atomic scattering codes with an order of magnitude less effort, for cases where one electron is assumed to ionize. With this method we can calculate angular distributions and electron energy spectra, which have not previously been available from realistic numerical simulations.

The basic properties of atoms, molecules, and solids are governed by ultrafast electron dynamics. Attosecond pulses bear the promise to resolve these electronic dynamics on their natural time scale, the atomic unit of time, which is 24 attoseconds. The high frequency of the pulses, however, means that in most of the experiments performed so far the electrons that are excited by attosecond pulses are directly moved into the ionization continuum, where they rapidly disperse [1,2]. More interesting dynamics arise when electrons are excited into bound [3] or autoionizing states [4]. Here we present a method to determine the dynamics of a bound wave packet excited by an attosecond pulse, while - for the first time - keeping track of its spectral content with high precision. The key idea is that coincident with the creation of the bound wave packet, we also launch a broad continuum wave packet (Fig. 1). This free wave packet serves as a reference when, after a variable delay, the bound wave packet is ionized by a 7 fs infrared laser pulse, locked in phase with the bound wave packet. The interference fringes observed in the photoelectron spectrum enable precise determination of the bound electron wave packet. As in Ramsey spectroscopy, the spectral precision is here set not by the bandwidth of the excitation pulse, but by the delay between the pump and probe pulses as well as the experimental energy resolution of the photoelectron spectrometer used.

We measured the frequency chirp rate of harmonics 13 to 23 in argon by cross-correlation with a short femtosecond probe pulse. We directly measured the negative chirp due to the atomic dipole phase and demonstrated that an additional chirp on the pump pulse was transferred to the qth harmonic as q times the fundamental chirp.