Nanoplasma resonance condition in the middle-infrared spectral range.

The resonance-absorption condition in the laser-nanoplasma interactions has been considered to follow the wavelength dependence of the critical plasma density. We experimentally demonstrate that this assumption fails in the middle-infrared spectral range, while it is valid for visible and near-infrared wavelengths. A thorough analysis supported by molecular dynamic (MD) simulations indicates that the observed transition in the resonance condition is caused by the reduction of the electron scattering rate and the associated increase of the cluster outer-ionization contribution. An expression for the nanoplasma resonance density is derived based on experimental results and MD simulations. The findings are important for a broad range of plasma experiments and applications, since the extension of the laser-plasma interaction studies to longer wavelengths has become increasingly topical.

Probing the interplay between geometric and electronic-structure features via high-harmonic spectroscopy.

We present molecular-frame measurements of the recombination dipole matrix element (RDME) in CO2, N2O, and carbonyl sulfide (OCS) molecules using high-harmonic spectroscopy. Both the amplitudes and phases of the RDMEs exhibit clear imprints of a two-center interference minimum, which moves in energy with the molecular alignment angle relative to the laser polarization. We find that whereas the angle dependence of this minimum is consistent with the molecular geometry in CO2 and N2O, it behaves very differently in OCS; in particular, the phase shift which accompanies the two-center minimum changes sign for different alignment angles. Our results suggest that two interfering structural features contribute to the OCS RDME, namely, (i) the geometrical two-center minimum and (ii) a Cooper-like, electronic-structure minimum associated with the sulfur end of the molecule. We compare our results to ab initio calculations using time-dependent density functional theory and present an empirical model that captures both the two-center and the Cooper-like interferences. We also show that the yield from unaligned samples of two-center molecules is, in general, reduced at high photon energies compared to aligned samples, due to the destructive interference between molecules with different alignments.

Wannier Representation of Intraband High-Order Harmonic Generation.

We investigate the harmonic generation induced by the interaction of a laser field with a solid target. The harmonic spectra is composed of the contribution of two processes interpreted as interband and intraband transitions. The interband process corresponds to the recombination from an upper band, populated during the laser interaction, to a lower band. The intraband process originates from nonlinear processes of the current in individual bands. In this Letter, we develop a theory based on Wannier states and reveal in depth the underlying physics of intraband dynamics. In particular, this approach highlights the determinant role of transitions between different lattice wells. Furthermore, our approach provides quantitative predictions concerning high-order harmonic energy cutoffs, harmonic yields, and emission times.

Tunable orbital angular momentum in high-harmonic generation.

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light-matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

Precise Access to the Molecular-Frame Complex Recombination Dipole through High-Harmonic Spectroscopy.

We report on spectral intensity and group delay measurements of the highest-occupied molecular-orbital (HOMO) recombination dipole moment of N_{2} in the molecular-frame using high harmonic spectroscopy. We take advantage of the long-wavelength 1.3 µm driving laser to isolate the HOMO in the near threshold region, 19-67 eV. The precision of our group delay measurements reveals previously unseen angle-resolved spectral features associated with autoionizing resonances, and allows quantitative comparison with cutting-edge correlated 8-channel photoionization dipole moment calculations.

Attosecond pulse shaping around a Cooper minimum.

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 straightforward 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.

Scaling of the low-energy structure in above-threshold ionization in the tunneling regime: theory and experiment.

A calculation of the second-order (rescattering) term in the S-matrix expansion of above-threshold ionization is presented for the case when the binding potential is the unscreened Coulomb potential. Technical problems related to the divergence of the Coulomb scattering amplitude are avoided in the theory by considering the depletion of the atomic ground state due to the applied laser field, which is well defined and does not require the introduction of a screening constant. We focus on the low-energy structure, which was observed in recent experiments with a midinfrared wavelength laser field. Both the spectra and, in particular, the observed scaling versus the Keldysh parameter and the ponderomotive energy are reproduced. The theory provides evidence that the origin of the structure lies in the long-range Coulomb interaction.

Explosions of xenon clusters in ultraintense femtosecond x-ray pulses from the LCLS free electron laser.

Explosions of large Xe clusters ( ~ 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.

Inelastic scattering of broadband electron wave packets driven by an intense midinfrared laser field.

Intense, 100 fs laser pulses at 3.2 and 3.6 µm are used to generate, by multiphoton ionization, broadband wave packets with up to 400 eV of kinetic energy and charge states up to Xe(+6). The multiple ionization pathways are well described by a white electron wave packet and field-free inelastic cross sections, averaged over the intensity-dependent energy distribution for (e, ne) electron impact ionization. The analysis also suggests a contribution from a 4d core excitation, or giant resonance, in xenon.

Unveiling and driving hidden resonances with high-fluence, high-intensity x-ray pulses.

We show that high fluence, high-intensity x-ray pulses from the world's first hard x-ray free-electron laser produce nonlinear phenomena that differ dramatically from the linear x-ray-matter interaction processes that are encountered at synchrotron x-ray sources. We use intense x-ray pulses of sub-10-fs duration to first reveal and subsequently drive the 1s↔2p resonance in singly ionized neon. This photon-driven cycling of an inner-shell electron modifies the Auger decay process, as evidenced by line shape modification. Our work demonstrates the propensity of high-fluence, femtosecond x-ray pulses to alter the target within a single pulse, i.e., to unveil hidden resonances, by cracking open inner shells energetically inaccessible via single-photon absorption, and to consequently trigger damaging electron cascades at unexpectedly low photon energies.

Nonlinear atomic response to intense ultrashort x rays.

The nonlinear absorption mechanisms of neon atoms to intense, femtosecond kilovolt x rays are investigated. The production of Ne(9+) is observed at x-ray frequencies below the Ne(8+), 1s(2) absorption edge and demonstrates a clear quadratic dependence on fluence. Theoretical analysis shows that the production is a combination of the two-photon ionization of Ne(8+) ground state and a high-order sequential process involving single-photon production and ionization of transient excited states on a time scale faster than the Auger decay. We find that the nonlinear direct two-photon ionization cross section is orders of magnitude higher than expected from previous calculations.

Auger electron angular distribution of double core-hole states in the molecular reference frame.

The Linac Coherent Light Source free electron laser is a source of high brightness x rays, 2×10(11) photons in a ∼5 fs pulse, that can be focused to produce double core vacancies through rapid sequential ionization. This enables double core vacancy Auger electron spectroscopy, an entirely new way to study femtosecond chemical dynamics with Auger electrons that probe the local valence structure of molecules near a specific atomic core. Using 1.1 keV photons for sequential x-ray ionization of impulsively aligned molecular nitrogen, we observed a rich single-site double core vacancy Auger electron spectrum near 413 eV, in good agreement with ab initio calculations, and we measured the corresponding Auger electron angle dependence in the molecular frame.

Time-resolved pump-probe experiments at the LCLS.

The first time-resolved x-ray/optical pump-probe experiments at the SLAC Linac Coherent Light Source (LCLS) used a combination of feedback methods and post-analysis binning techniques to synchronize an ultrafast optical laser to the linac-based x-ray laser. Transient molecular nitrogen alignment revival features were resolved in time-dependent x-ray-induced fragmentation spectra. These alignment features were used to find the temporal overlap of the pump and probe pulses. The strong-field dissociation of x-ray generated quasi-bound molecular dications was used to establish the residual timing jitter. This analysis shows that the relative arrival time of the Ti:Sapphire laser and the x-ray pulses had a distribution with a standard deviation of approximately 120 fs. The largest contribution to the jitter noise spectrum was the locking of the laser oscillator to the reference RF of the accelerator, which suggests that simple technical improvements could reduce the jitter to better than 50 fs.

Femtosecond electronic response of atoms to ultra-intense X-rays.

An era of exploring the interactions of high-intensity, hard X-rays with matter has begun with the start-up of a hard-X-ray free-electron laser, the Linac Coherent Light Source (LCLS). Understanding how electrons in matter respond to ultra-intense X-ray radiation is essential for all applications. Here we reveal the nature of the electronic response in a free atom to unprecedented high-intensity, short-wavelength, high-fluence radiation (respectively 10(18) W cm(-2), 1.5-0.6 nm, approximately 10(5) X-ray photons per A(2)). At this fluence, the neon target inevitably changes during the course of a single femtosecond-duration X-ray pulse-by sequentially ejecting electrons-to produce fully-stripped neon through absorption of six photons. Rapid photoejection of inner-shell electrons produces 'hollow' atoms and an intensity-induced X-ray transparency. Such transparency, due to the presence of inner-shell vacancies, can be induced in all atomic, molecular and condensed matter systems at high intensity. Quantitative comparison with theory allows us to extract LCLS fluence and pulse duration. Our successful modelling of X-ray/atom interactions using a straightforward rate equation approach augurs favourably for extension to complex systems.

Attosecond synchronization of high-order harmonics from midinfrared drivers.

The group delay dispersion, also known as the attochirp, of high-order harmonics generated in gases has been identified as the main intrinsic limitation to the duration of Fourier-synthesized attosecond pulses. Theory implies that the attochirp, which is inversely proportional to the laser wavelength, can be decreased at longer wavelength. Here we report the first measurement of the wavelength dependence of the attochirp using an all-optical, in situ method [N. Dudovich, Nature Phys. 2, 781 (2006)10.1038/nphys434]. We show that a 2 microm driving wavelength reduces the attochirp with respect to 0.8 microm at comparable intensities.

Intense self-compressed, self-phase-stabilized few-cycle pulses at 2 microm from an optical filament.

We report the compression of intense, carrier-envelope phase stable mid-IR pulses down to few-cycle duration using an optical filament. A filament in xenon gas is formed by using self-phase stabilized 330 microJ 55 fs pulses at 2 microm produced via difference-frequency generation in a Ti:sapphire-pumped optical parametric amplifier. The ultrabroadband 2 microm carrier-wavelength output is self-compressed below 3 optical cycles and has a 270 microJ pulse energy. The self-locked phase offset of the 2 microm difference-frequency field is preserved after filamentation. This is to our knowledge the first experimental realization of pulse compression in optical filaments at mid-IR wavelengths (lambda>0.8 microm).

Scaling of wave-packet dynamics in an intense midinfrared field.

A theoretical investigation is presented that examines the wavelength scaling from near-visible (0.8 micro m) to midinfrared (2 micro m) of the photoelectron distribution and high harmonics generated by a "single" atom in an intense electromagnetic field. The calculations use a numerical solution of the time-dependent Schrödinger equation (TDSE) in argon and the strong-field approximation in helium. The scaling of electron energies (lambda2), harmonic cutoff (lambda2), and attochirp (lambda -1) agree with classical mechanics, but it is found that, surprisingly, the harmonic yield follows a lambda -(5-6) scaling at constant intensity. In addition, the TDSE results reveal an unexpected contribution from higher-order returns of the rescattering electron wave packet.

High-energy cutoff in the spectrum of strong-field nonsequential double ionization.

Electron energy distributions of singly and doubly ionized helium in an intense 390 nm laser field have been measured at two intensities (0.8 PW/cm2 and 1.1 PW/cm2, where PW is defined as 10(15) W/cm2). Numerical solutions of the full-dimensional time-dependent helium Schrödinger equation show excellent agreement with the experimental measurements. The high-energy portion of the two-electron energy distributions reveals an unexpected 5U(p) cutoff for the double ionization (DI) process and leads to a proposed model for DI below the quasiclassical threshold.

Enhanced high harmonic generation from an optically prepared excited medium.

We investigate high harmonics generated from rubidium atoms irradiated simultaneously by an intense 3.5 microm fundamental field and a weak cw diode laser. When 5p, 5d, and 4d excited states are populated through cascade excitation or deexcitation, orders-of-magnitude increases in harmonic yield as compared with the ground state are observed. It appears that, quite unexpectedly, the population accumulated in the 4d state alone is responsible for the observed enhancement.

Multiphoton double ionization via field-independent resonant excitation.

The double ionization of xenon in the multiphoton regime has been studied at two wavelengths (0.77 and 0.79 microm) using an electron-ion coincidence technique and an intensity binned ion ratio method. Sharp resonant structures in the electron energy distribution correlated with the doubly charged ion, as well as a wavelength dependence of the Xe(2+)/Xe(+) ratio provides new insights. A mechanism involving the shelving of population in Rydberg states followed by excitation of a core electron is proposed.