Propagation effects in polarization-gated attosecond soft-X-ray pulse generation.

Accurate estimation of the duration of soft-x-ray pulses from high-harmonic generation (HHG) remains challenging given their higher photon energies and broad spectral bandwidth. The carrier-envelope-phase (CEP) dependence of generated soft-x-ray spectra is indicative of attosecond pulse generation, but advanced simulations are needed to infer the pulse duration from such data. Here, we employ macroscopic propagation simulations to reproduce experimental polarization-gated CEP-dependent soft-x-ray spectra. The simulations indicate chirped pulses, which we theoretically find to be compressible in hydrogen plasmas, suggesting this as a viable compression scheme for broadband soft-x-rays from HHG.

Orbital perspective on high-harmonic generation from solids.

High-harmonic generation in solids allows probing and controlling electron dynamics in crystals on few femtosecond timescales, paving the way to lightwave electronics. In the spatial domain, recent advances in the real-space interpretation of high-harmonic emission in solids allows imaging the field-free, static, potential of the valence electrons with picometer resolution. The combination of such extreme spatial and temporal resolutions to measure and control strong-field dynamics in solids at the atomic scale is poised to unlock a new frontier of lightwave electronics. Here, we report a strong intensity-dependent anisotropy in the high-harmonic generation from ReS2 that we attribute to angle-dependent interference of currents from the different atoms in the unit cell. Furthermore, we demonstrate how the laser parameters control the relative contribution of these atoms to the high-harmonic emission. Our findings provide an unprecedented atomic perspective on strong-field dynamics in crystals, revealing key factors to consider in the route towards developing efficient harmonic emitters.

D3+ formation through photoionization of the molecular D2-D2 dimer.

The H2-H2 molecular dimer is of fundamental importance in the study of chemical interactions because of its unique bonding properties and its ability to model more complex systems. The trihydrogen cation H3+ is also a key intermediate in a range of chemical processes in interstellar environments, such as the formation of various organic molecules and early stars. However, the unexpected high abundance of H3+ in molecular clouds remains challenging to explain. Here using near-infrared, femtosecond laser pulses and coincidence momentum imaging, we find that the dominant channel after photoionization of a deuterium molecular dimer (D2-D2) is the ejection of a deuterium atom within a few hundred femtoseconds, leading to the formation of D3+. The formation mechanism is supported and well-reproduced by ab initio molecular dynamics simulations. This pathway of D3+ formation from ultracold D2-D2 gas may provide insights into the high abundance of H3+ in the interstellar medium.

Chiral high-harmonic generation and spectroscopy on solid surfaces using polarization-tailored strong fields.

Strong-field methods in solids enable new strategies for ultrafast nonlinear spectroscopy and provide all-optical insights into the electronic properties of condensed matter in reciprocal and real space. Additionally, solid-state media offers unprecedented possibilities to control high-harmonic generation using modified targets or tailored excitation fields. Here we merge these important points and demonstrate circularly-polarized high-harmonic generation with polarization-matched excitation fields for spectroscopy of chiral electronic properties at surfaces. The sensitivity of our approach is demonstrated for structural helicity and termination-mediated ferromagnetic order at the surface of silicon-dioxide and magnesium oxide, respectively. Circularly polarized radiation emanating from a solid sample now allows to add basic symmetry properties as chirality to the arsenal of strong-field spectroscopy in solids. Together with its inherent temporal (femtosecond) resolution and non-resonant broadband spectrum, the polarization control of high harmonics from condensed matter can illuminate ultrafast and strong field dynamics of surfaces, buried layers or thin films.

Clocking Enhanced Ionization of Hydrogen Molecules with Rotational Wave Packets.

Laser-induced rotational wave packets of H_{2} and D_{2} molecules were experimentally measured in real time by using two sequential 25-fs laser pulses and a reaction microscope. By measuring the time-dependent yields of the above-threshold dissociation and the enhanced ionization of the molecule, we observed a few-femtosecond time delay between the two dissociation channels for both H_{2} and D_{2}. The delay was interpreted and reproduced by a classical model that considers enhanced ionization and thus additional interaction within the laser pulse. We demonstrate that by accurately measuring the phase of the rotational wave packet in hydrogen molecules we can resolve dissociation dynamics which is occurring within a fraction of a molecular rotation. Such a rotational clock is a general concept applicable to sequential fragmentation processes in other molecules.

Multiphoton laser-induced confined chemical changes in polymer films.

We report ultrafast-laser-induced photochemical, structural, and morphological changes in a polyimide film irradiated at the polymer-glass interface in back-incident geometry. Back-illumination creates locally hot material at the interface leading to a confined photochemical change at the interface and a morphological change through a blister formation. The laser-induced photochemical changes in polyimide resulted in new absorption and luminescence properties in the visible region. The laser-treated polyimide exhibited photoluminescence anisotropy resulting from formation of ordered polymer upon irradiation by linearly polarized ultrashort laser pulses. Confocal fluorescence microscopy resulted in similar observations to the bulk. Reflection-absorption infrared spectroscopy and X-ray photoelectron spectroscopy together indicated confinement of laser-induced chemical changes at the interface.

Experimental Separation of Subcycle Ionization Bursts in Strong-Field Double Ionization of H_{2}.

We report on the unambiguous observation of the subcycle ionization bursts in sequential strong-field double ionization of H_{2} and their disentanglement in molecular frame photoelectron angular distributions. This observation was made possible by the use of few-cycle laser pulses with a known carrier-envelope phase, in combination with multiparticle coincidence momentum imaging. The approach demonstrated here will allow sampling of the intramolecular electron dynamics and the investigation of charge-state-specific Coulomb distortions on emitted electrons in polyatomic molecules.

Subfemtosecond Tracing of Molecular Dynamics during Strong-Field Interaction.

We introduce and experimentally demonstrate a method where the two intrinsic timescales of a molecule, the slow nuclear motion and the fast electronic motion, are simultaneously measured in a photoelectron photoion coincidence experiment. In our experiment, elliptically polarized, 750 nm, 4.5 fs laser pulses were focused to an intensity of 9×10^{14} W/cm^{2} onto H_{2}. Using coincidence imaging, we directly observe the nuclear wave packet evolving on the 1sσ_{g} state of H_{2}^{+} during its first round-trip with attosecond temporal and picometer spatial resolution. The demonstrated method should enable insight into the first few femtoseconds of the vibronic dynamics of ionization-induced unimolecular reactions of larger molecules.

Disentangling Intracycle Interferences in Photoelectron Momentum Distributions Using Orthogonal Two-Color Laser Fields.

We use orthogonally polarized two-color (OTC) laser pulses to separate quantum paths in the multiphoton ionization of Ar atoms. Our OTC pulses consist of 400 and 800 nm light at a relative intensity ratio of 10∶1. We find a hitherto unobserved interference in the photoelectron momentum distribution, which exhibits a strong dependence on the relative phase of the OTC pulse. Analysis of model calculations reveals that the interference is caused by quantum pathways from nonadjacent quarter cycles.

Tailored semiconductors for high-harmonic optoelectronics.

The advent of high-harmonic generation in gases 30 years ago set the foundation for attosecond science and facilitated ultrafast spectroscopy in atoms, molecules, and solids. We explore high-harmonic generation in the solid state by means of nanostructured and ion-implanted semiconductors. We use wavelength-selective microscopic imaging to map enhanced harmonic emission and show that the generation medium and the driving field can be locally tailored in solids by modifying the chemical composition and morphology. This enables the control of high-harmonic technology within precisely engineered solid targets. We demonstrate customized high-harmonic wave fields with wavelengths down to 225 nanometers (ninth-harmonic order of 2-micrometer laser pulses) and present an integrated Fresnel zone plate target in silicon, which leads to diffraction-limited self-focusing of the generated harmonics down to 1-micrometer spot sizes.

Phase Structure of Strong-Field Tunneling Wave Packets from Molecules.

We study the phase structure of the tunneling wave packets from strong-field ionization of molecules and present a molecular quantum-trajectory Monte Carlo model to describe the laser-driven dynamics of photoelectron momentum distributions of molecules. Using our model, we reproduce and explain the alignment-dependent molecular frame photoelectron spectra of strong-field tunneling ionization of N_{2} reported by M. Meckel et al. [Nat. Phys. 10, 594 (2014)]. In addition to modeling the low-energy photoelectron angular distributions quantitatively, we extract the phase structure of strong-field molecular tunneling wave packets, shedding light on its physical origin. The initial phase of the tunneling wave packets at the tunnel exit depends on both the initial transverse momentum distribution and the molecular internuclear distance. We further show that the ionizing molecular orbital has a critical effect on the initial phase of the tunneling wave packets. The phase structure of the photoelectron wave packet is a key ingredient for modeling strong-field molecular photoelectron holography, high-harmonic generation, and molecular orbital imaging.

Probing Molecular Dynamics by Laser-Induced Backscattering Holography.

We use differential holography to overcome the forward scattering problem in strong-field photoelectron holography. Our differential holograms of H_{2} and D_{2} molecules exhibit a fishbonelike structure, which arises from the backscattered part of the recolliding photoelectron wave packet. We demonstrate that the backscattering hologram can resolve the different nuclear dynamics between H_{2} and D_{2} with subangstrom spatial and subcycle temporal resolution. In addition, we show that attosecond electron dynamics can be resolved. These results open a new avenue for ultrafast studies of molecular dynamics in small molecules.

Subcycle control of electron-electron correlation in double ionization.

Double ionization of neon with orthogonally polarized two-color (OTC) laser fields is investigated using coincidence momentum imaging. We show that the two-electron emission dynamics in nonsequential double ionization can be controlled by tuning the subcycle shape of the electric field of the OTC pulses. We demonstrate experimentally switching from correlated to anticorrelated two-electron emission, and control over the directionality of the two-electron emission. Simulations based on a semiclassical trajectory model qualitatively explain the experimental results by a subcycle dependence of the electron recollision time on the OTC field shape.