Attosecond delays in X-ray molecular ionization.

The photoelectric effect is not truly instantaneous but exhibits attosecond delays that can reveal complex molecular dynamics. Sub-femtosecond-duration light pulses provide the requisite tools to resolve the dynamics of photoionization. Accordingly, the past decade has produced a large volume of work on photoionization delays following single-photon absorption of an extreme ultraviolet photon.

Sub-two-cycle gigawatt-peak-power LWIR OPA for ultrafast nonlinear spectroscopy of condensed state materials.

The application of high-power, few-cycle, long-wave infrared (LWIR, 8-20 µm) pulses in strong-field physics is largely unexplored due to the lack of suitable sources. However, the generation of intense pulses with >6 µm wavelength range is becoming increasingly feasible with the recent advances in high-power ultrashort lasers in the middle-infrared range that can serve as a pump for optical parametric amplifiers (OPA). Here we experimentally demonstrate the feasibility of this approach by building an OPA pumped at 2.

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.

High-order harmonic generation from a thin film crystal perturbed by a quasi-static terahertz field.

Studies of laser-driven strong field processes subjected to a (quasi-)static field have been mainly confined to theory. Here we provide an experimental realization by introducing a bichromatic approach for high harmonic generation (HHG) in a dielectric that combines an intense 70 femtosecond duration mid-infrared driving field with a weak 2 picosecond period terahertz (THz) dressing field. We address the physics underlying the THz field induced static symmetry breaking and its consequences on the efficient production/suppression of even-/odd-order harmonics, and demonstrate the ability to probe the HHG dynamics via the modulation of the harmonic distribution.

Attochemistry Regulation of Charge Migration.

Charge migration (CM) is a coherent attosecond process that involves the movement of localized holes across a molecule. To determine the relationship between a molecule's structure and the CM dynamics it exhibits, we perform systematic studies of para-functionalized bromobenzene molecules (X-CH-R) using real-time time-dependent density functional theory. We initiate valence-electron dynamics by emulating rapid strong-field ionization leading to a localized hole on the bromine atom.

Investigation of Interferences in Carbon Dioxide through Multidimensional Molecular-Frame High-Harmonic Spectroscopy.

We present molecular-frame high-harmonic spectroscopic measurements of the spectral intensity and group delay of carbon dioxide. Using four different driving wavelengths and a range of intensities at each wavelength for high-harmonic generation, we observe a well-characterized minimum in the harmonic emission that exhibits both a wavelength and intensity dependence. Using the intensity dependence at each driving wavelength, we classify the minimum as due to either a structural two-center interference or dynamic multichannel interference, consistent with previous literature.

Attosecond coherent electron motion in Auger-Meitner decay.

In quantum systems, coherent superpositions of electronic states evolve on ultrafast time scales (few femtoseconds to attoseconds; 1 attosecond = 0.001 femtoseconds = 10 seconds), leading to a time-dependent charge density. Here we performed time-resolved measurements using attosecond soft x-ray pulses produced by a free-electron laser, to track the evolution of a coherent core-hole excitation in nitric oxide.

Two-dimensional retrieval methods for ultrafast imaging of molecular structure using laser-induced electron diffraction.

Molecular structural retrieval based on electron diffraction has been proposed to determine the atomic positions of molecules with sub-angstrom spatial and femtosecond temporal resolutions. Given its success on small molecular systems, in this work, we point out that the accuracy of structure retrieval is constrained by the availability of a wide range of experimental data in the momentum space in all molecular systems. To mitigate the limitations, for laser-induced electron diffraction, here we retrieve molecular structures using two-dimensional (energy and angle) electron momentum spectra in the laboratory frame for a number of small molecular systems, which have previously been studied with 1D methods.

Molecular Modes of Attosecond Charge Migration.

First-principles calculations are employed to elucidate the modes of attosecond charge migration (CM) in halogenated hydrocarbon chains. We use constrained density functional theory (DFT) to emulate the creation of a localized hole on the halogen and follow the subsequent dynamics via time-dependent DFT. We find low-frequency CM modes (∼1  eV) that propagate across the molecule and study their dependence on length, bond order, and halogenation.

Universal High-Energy Photoelectron Emission from Nanoclusters Beyond the Atomic Limit.

Rescattering by electrons on classical trajectories is central to understand photoelectron and high-harmonic emission from isolated atoms or molecules in intense laser pulses. By controlling the cluster size and the quiver amplitude of electrons, we demonstrate how rescattering influences the energy distribution of photoelectrons emitted from noble gas nanoclusters. Our experiments reveal a universal dependence of photoelectron energy distributions on the cluster size when scaled by the field driven electron excursion, establishing a unified rescattering picture for extended systems with the known atomic dynamics as the limit of zero extension.

Attosecond transient absorption spooktroscopy: a ghost imaging approach to ultrafast absorption spectroscopy.

The recent demonstration of isolated attosecond pulses from an X-ray free-electron laser (XFEL) opens the possibility for probing ultrafast electron dynamics at X-ray wavelengths. An established experimental method for probing ultrafast dynamics is X-ray transient absorption spectroscopy, where the X-ray absorption spectrum is measured by scanning the central photon energy and recording the resultant photoproducts. The spectral bandwidth inherent to attosecond pulses is wide compared to the resonant features typically probed, which generally precludes the application of this technique in the attosecond regime.

Disentangling Spectral Phases of Interfering Autoionizing States from Attosecond Interferometric Measurements.

We have determined spectral phases of Ne autoionizing states from extreme ultraviolet and midinfrared attosecond interferometric measurements and ab initio full-electron time-dependent theoretical calculations in an energy interval where several of these states are coherently populated. The retrieved phases exhibit a complex behavior as a function of photon energy, which is the consequence of the interference between paths involving various resonances. In spite of this complexity, we show that phases for individual resonances can still be obtained from experiment by using an extension of the Fano model of atomic resonances.

High-harmonic spectroscopy of transient two-center interference calculated with time-dependent density-functional theory.

We demonstrate high-harmonic spectroscopy in many-electron molecules using time-dependent density-functional theory. We show that a weak attosecond-pulse-train ionization seed that is properly synchronized with the strong driving mid-infrared laser field can produce experimentally relevant high-harmonic generation (HHG) signals, from which we extract both the spectral amplitude and the target-specific phase (group delay). We also show that further processing of the HHG signal can be used to achieve molecular-frame resolution, i.

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 CO, NO, 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 CO and NO, it behaves very differently in OCS; in particular, the phase shift which accompanies the two-center minimum changes sign for different alignment angles.

Diffractive Imaging of C_{60} Structural Deformations Induced by Intense Femtosecond Midinfrared Laser Fields.

Theoretical studies indicated that C_{60} exposed to linearly polarized intense infrared pulses undergoes periodic cage structural distortions with typical periods around 100 fs (1  fs=10^{-15}  s). Here, we use the laser-driven self-imaging electron diffraction technique, previously developed for atoms and small molecules, to measure laser-induced deformation of C_{60} in an intense 3.6  μm laser field.

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 roles of photo-carrier doping and driving wavelength in high harmonic generation from a semiconductor.

High-harmonic generation from gases produces attosecond bursts and enables high-harmonic spectroscopy to explore electron dynamics in atoms and molecules. Recently, high-harmonic generation from solids has been reported, resulting in novel phenomena and unique control of the emission, absent in gas-phase media. Here we investigate high harmonics from semiconductors with controllable induced photo-carrier densities, as well as the driving wavelengths.

High-order harmonic generations in intense MIR fields by cascade three-wave mixing in a fractal-poled LiNbO photonic crystal.

We report on the generation of harmonic-like photon upconversion in a LiNbO-based nonlinear photonic crystal by mid-infrared (MIR) femtosecond laser pulses. We study below bandgap harmonics of various driver wavelengths, reaching up to the 11th order at 4 μm driver with 13% efficiency. We compare our results to numerical simulations based on two mechanisms: cascade three-wave mixing and non-perturbative harmonic generation, both of which include quasi-phase matching.

Tunable mid-infrared source of light carrying orbital angular momentum in the femtosecond regime.

We report on a tunable intense femtosecond mid-infrared (mid-IR) light source carrying orbital angular momentum (OAM). Our setup is based on an optical parametric amplification system with an 800 nm pump shaped with a spiral phase plate. We confirm the anisotropic OAM transfer from the pump to the idler through stimulated difference frequency generation by measuring the diffraction patterns of a triangular aperture illuminated by the signal, pump, and idler beams.

High-energy mid-infrared sub-cycle pulse synthesis from a parametric amplifier.

High-energy phase-stable sub-cycle mid-infrared pulses can provide unique opportunities to explore phase-sensitive strong-field light-matter interactions in atoms, molecules and solids. At the mid-infrared wavelength, the Keldysh parameter could be much smaller than unity even at relatively modest laser intensities, enabling the study of the strong-field sub-cycle electron dynamics in solids without damage. Here we report a high-energy sub-cycle pulse synthesiser based on a mid-infrared optical parametric amplifier and its application to high-harmonic generation in solids.

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.

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.

Laser induced periodic surface structure formation in germanium by strong field mid IR laser solid interaction at oblique incidence.

Laser induced periodic surface structures (LIPSS or ripples) were generated on single crystal germanium after irradiation with multiple 3 µm femtosecond laser pulses at a 45° angle of incidence. High and low spatial frequency LIPSS (HSFL and LSFL, respectively) were observed for both s- and p-polarized light. The measured LSFL period for p-polarized light was consistent with the currently established LIPSS origination model of coupling between surface plasmon polaritons (SPP) and the incident laser pulses.