Interaction of Optical Vortices with Atoms and Molecules

Over the last decades, the study of photoionization of atoms and molecules has been an active field in physics, and quite a few theoretical methods have been proposed and applied successfully. Most of these studies were carried considering interaction with photons—specially described by plane waves or short laser pulses—within the dipolar approximation only (transferring only one unit of angular momenta). However, in recent years, light beams carrying orbital angular momentum (OAM), such as Laguerre-Gaussian beams, become available, allowing one to go beyond the dipolar approximation. Then, some extensions to the current theoretical methods are needed, in order to explore this new framework in nonlinear and quantum optics.

In this contribution we explore the implementation of the Sturmian approach, based on the use of generalized Sturmian functions, to the study of ionization of atoms and molecules by the interaction with electromagnetic fields carrying OAM. We investigate the new emergent selection rules and the different mechanisms of transfer of angular momenta with the target.

Attosecond Electron Scattering in Dielectrics

Scattering of electrons in dielectric materials is at the heart of laser nanomachining, light-driven electronics, and radiation damage. Knowledge of low-energy electron transport involving elastic and inelastic electron scattering is key to the accurate prediction of clustered damage following high-energy radiation exposure. Here, we report real-time access to electron scattering in dielectrics. Photoelectrons are generated inside isolated dielectric nanoparticles by an attosecond extreme ultraviolet pulse and streaked by a synchronized intense near-infrared field. We develop the theoretical framework for attosecond streaking in dielectrics and identify that the presence of the internal field cancels the influence of elastic scattering, enabling the selective characterization of the inelastic scattering time. The metrology is demonstrated on silica, where an inelastic mean free path is extracted for 20-30 eV, and we show that it is applicable for a broad range of energies and dielectric solids and liquids including those relevant to tissue- damage after irradiation.

High harmonic interferometry of the Lorentz force in strong mid-infrared laser fields

High harmonic radiation is a useful source of short pulses of high-frequency XUV radiation, and the cutoff frequency should increase when the driving field is stronger and has a longer wavelength. Unfortunately, in those regimes the magnetic field of the driver acts to deflect the electron excursion and prevent its recollision with the core, thus inhibiting harmonic emission, as a result of a breakdown of the dipole approximation in the long-wavelength regime.

We show that the combination of two non-collinear counter-rotating circularly polarized beams produces a small forwards ellipticity, and that this can be used to probe, measure, control, and cancel the effect of this magnetic field. Thus we obtain a flexible scheme to re-enable harmonic emission deep in the long-wavelength and strong field regimes. We show, moreover, that the beam configuration can be used with currently-available sources to demonstrate clear traces, in the form of even harmonics, of the breakdown of the dipole approximation.

The circular w+2w scheme: symmetry breaking and ways to control the ellipticity of attosecond bursts

Generation of coherent light sources with attosecond duration and controllable ellipticity will enable studies on chiral-sensitive systems at ultrafast time-scales. An elegant approach to the generation of such pulses consists of combining a circularly polarized fundamental field with a counter-rotating second harmonic. This scheme leads to harmonic peaks at (3N+1) and (3N+2) lines, with the helicity of the fundamental field and the second harmonic, respectively, while 3N harmonics are absent due to symmetry. Suppression of one of the lines will thus yield highly elliptical pulses. Several studies have investigated this possibility, but control over such ellipticity is still to be achieved. Moreover, recent experiments have reported the appearance of forbidden 3N harmonics even for perfectly three-foil-symmetric pulses, implying a breaking of the symmetry that is yet to be explained.

In this work, we outline the physical mechanism responsible for the different strengths of the lines observed in the harmonic spectrum and the reasons behind the breaking of the symmetry in the system. We do so by solving the time-dependent Schrödinger equation in the single-active electron approximation for both helium and neon, and compare our results to experiments. Additionally, we derive ionization and recombination propensity rules in the harmonic process, and show how they can be used to control the ellipticity of the generated attosecond bursts.

Illuminating Molecular Symmetries with Bicircular High-Order Harmonic Generation

I will present a general theory of bicircular high-order-harmonic generation from N-fold rotationally symmetric molecules that has recently been formulated. Using a rotating frame of reference it is possible to predict the complete structure of the high-order-harmonic spectra for arbitrary driving frequency ratios and molecular symmetries can be directly identified from the high-harmonic signal. These findings reveal that a characteristic fingerprint of rotational molecular symmetries can be universally observed in the ultrafast response of molecules to strong bicircular fields.

Dissociation of HeH+ in intense laser pulses

HeH+ is the simplest asymmetric molecule and therefore an ideal candidate to study the effects of short laser pulses on dissociation and ionization. Short laser pulses themselves show a significant asymmetry when the CEP is varied. We’ve tuned a 1D single-active-electron model to reproduce some physical key properties and use it to calculate dissociation properties as a function of laser parameters by solving the TDSE. The (1D) nuclear dynamic is completely included.

Attosecond-streaking spectroscopy on a liquid-water microjet

Attosecond-streaking spectroscopy has given real-time access to photoionization delays of atoms in the gas phase and the additional effects of electron transport processes through atomic layers and interfaces of solid-state systems. Here, we report on the first attosecond-streaking experiments on liquid samples. We have realized attosecond-streaking photoelectron spectroscopy on water in the gas and liquid phases using a liquid microjet.

In our experiments, a carrier-envelope-phase-stabilized (~200 mrad rms), sub-5 fs, Ti:Sapphire laser system (4 kHz, 1.2 mJ, 790 nm) is used to generate isolated attosecond pulses by intensity gating, centered at 90 eV (~5 eV FWHM, 455 as Fourier transform-limited). The residual NIR and the generated as-XUV pulses are focused by a two-component mirror assembly onto the liquid microjet (25 μm diameter). Scanning the delay between the XUV pump and the NIR streaking pulse with a piezo linear stage allows us to measure time-dependent photoelectron spectra by means of a field-free time-of-flight (TOF) spectrometer. Successive measurements on gas-phase water evaporating from the liquid microjet and from liquid water inside the jet are performed by translating the jet away from the entrance of the TOF.

The measurements on gas-phase water molecules provide effective photoionization delays between the outer- (1b1, 3a1, 1b2) and inner-valence (2a1) shells. These delays contain information on the photoionization dynamics of the molecule and, possibly, electron-correlation phenomena that are known to play a role in inner-valence ionization. The measurements on liquid water additionally provide insight into the transport of electrons through liquid water on the attosecond time scale, including elastic and inelastic scattering of electrons with liquid-phase water molecules.

Recollision with spin-polarized electrons in tailored laser fields

Ionization of noble gases by strong infrared circularly-polarized laser pulses can produce electron currents with a controllable degree of spin polarization. Spin polarization arises as a result of (1) entanglement between the emitted electron and the parent ion, and (2) sensitivity of ionization to the sense of electron rotation in the initial state. The use of two-color counter- rotating bi-circular fields opens new opportunities for introducing the spin degree of freedom into attosecond science, since the liberated electron can be driven back towards the parent ion within one optical cycle. We show that electrons recolliding with the ionic core upon tunnel ionization of xenon atoms driven by strong bi-circular fields are spin polarized and that their degree of polarization depends strongly on the recollision time (energy). We have found that the level of polarization can be modified by tailoring the driving fields, opening the door for attosecond control of spin-resolved dynamics.

Laser-subcycle control of sequential double-ionization dynamics of Helium

We investigate sequential double-ionization of helium by intense near- circularly polarized few-cycle laser pulses using a classical trajectory-based model with two independent electrons. Simulated He2 + ion momentum distributions are compared to those obtained in recent benchmark experiments. By comparison to semi-classical trajectory simulations we succeed in assigning the corresponding structures in the measured distributions to certain two-electron emission dynamics. We investigate the influence of a number of pulse parameters such as peak intensity, carrier- envelope phase, pulse duration and second- and third-order spectral phase on the shape of the ion momentum distributions. Good agreement is found in the main features of these distributions and of their dependence on the laser pulse duration, peak intensity and carrier-envelope phase. We demonstrate that the sequential double ionization dynamics can be sensitively controlled with the pulse duration and the laser peak intensity. Furthermore, we observe that for explaining certain fine-scale features observed in the measurement, it becomes important to consider subtle timing-variations in the two-electron emissions introduced by small values of chirp. This result highlights the possibility of measuring and controlling multi- electron dynamics on the attosecond time-scale by fine-tuning the field- evolution of intense close to single-cycle laser pulses.

Optimization of strong laser field-free alignment using tailored light fields

Alignment of molecules with respect to the laboratory fixed frame enables the realization of a large variety of experiments such as the determination of molecular frame photoangular distribution (MFPAD's) or laser induced electron diffraction (LIED) where typically a strong degree of alignment is needed. We present a combined theoretical and experimental effort to optimise the degree of laser field-free alignment of molecules in the gas phase. We start by solving the time-dependent rotational Schrödinger equation coupled to a non-resonant laser field and a static electric field and use an iterative learning-loop algorithm to determine the ideal pulse shape that optimises the degree of alignment. These calculations serve as a guide to complement the experiments where the alignment laser pulse form is optimally tailored. We discuss the simulation results and the experimental realization of two-pulse impulsive alignment on the example of the linear molecule carbonyl sulfide (OCS) and give an outlook for the use of pulse shaping techniques to achieve strongly aligned asymmetric top molecules.

A heuristical model for strong-field ionization of diatomic molecules

We present a model describing the interaction of neutral diatomic molecules with strong laser fields in terms of a two-level system coupled to a free electron. The model is applied to ionization of Neon-2 molecules and compared with recent measurements by the Dörner-Group from Frankfurt.

Single-photon ionization in intense, stochastically fluctuating pulses

High intensities in laser-matter interaction drive nonlinear processes. Whereas at low frequencies thereby multi-photon absorption and above- threshold ionization emerges, in the case of high frequencies single-photon absorption remains prevailing. However, multiple absorption and emission of photons renders this single-photon ionization sensitive to energy and shape of the laser pulse. This becomes relevant for intense, fluctuating pulses as generated in existing and upcoming free-electron laser sources. We study their effect on the ionization of a model atom numerically and formulate suitable parameters to characterise the evolution from the linear response at low intensity to the intricate dynamics at high intensities.

Free-electron quantum optics

In the last decade, electron microscopy, one of the most powerful and versatile techniques for the study of materials properties on atomic length scales, has been propelled to the realm of ultrafast dynamics. In an ultrafast transmission electron microscope (UTEM), ultrashort laser pulses are employed to pump a sample, which is then probed at a later time by ultrashort electron pulses. Besides time-resolved measurements of nanoscale dynamics, this allows for a quantum coherent manipulation of free electron beams with optical near-fields. The otherwise forbidden inelastic scattering between the free electrons and light is enabled by near-field confinement. Traversal of an optical near-field imprints a sinusoidal phase modulation on the electron wavefunction, such that the energetically narrow incident electron beam develops photon sidebands in its kinetic energy spectrum.

In this contribution, I will demonstrate coherent control schemes using free electrons. First, an electron-light interferometer is realized, in which the momentum distribution generated by a first near-field is further broadened or recompressed to the initial distribution depending on the relative near-field phase. Second, phase-controlled two-color excitation allows for tailoring complex phase modulations and strongly asymmetric electron energy spectra. These results open up a new path in the active manipulation of free electron beams, with the opportunity to generate specific transverse profiles and arbitrary temporal electron pulse structures. In the future, such schemes may enable new forms of time-resolved electron microscopy with sub-femtosecond precision.

Velocity map imaging of scattering dynamics in orthogonal two color fields

In strong-field ionization processes orthogonal two-color laser fields are frequently used for controlling sub-cycle electron dynamics via the relative phase of the laser fields. Here this technique is applied to velocity map imaging spectroscopy by using an unconventional orientation with the polarization of the ionizing laser field perpendicular to and the steering field parallel to the detector surface. Phase-dependent measurements of the photoelectron momentum distribution of Neon and Xenon demonstrate control over direct and rescattered electrons. The results are compared with semi-classical calculations in three dimensions including elastic scattering at different orders of return and with three-dimensional time-dependent Schrödinger equation calculations.