Sub-Cycle Control of Strong-Field Processes on the Attosecond Timescale

This PhD thesis deals with the sub-cycle nature of ultrafast phenomena that occur in strong-field light–matter interactions. As it is of interest to control these phenomena, we must understand them in order to manipulate them. The tools at our disposal are intense laser pulses of short duration, and the systems we study are atoms. A host of exotic phenomena may occur in strong-field light–matter interaction, such as high-order harmonic generation and above-threshold ionization. These processes exhibit aspects of both quantum mechanics and classical mechanics, in a fascinating blend.An important part of the work described in this thesis concerns the quantum paths of the electrons involved in these processes. The link between their journey and the time at which their journey begins is examined in a variety of ways. One property that quantum mechanical particles do not share with their classical counterparts is that the former may take a multitude of paths to reach their final destination. Furthermore, these paths may interfere such that the probability of detecting the particle is enhanced, suppressed, or sometimes even completely cancelled out

Studying Electron Dynamics using Attosecond Streaking

In this thesis, a program was implemented to study the electron dynamics of photoionization. These dynamics are probed by a streaking infrared field, that modulates the electrons' trajectories. Analytical quantum mechanical calculations for such systems are impossible without approximations or atoms more complex than hydrogen. However, using classical mechanics and Monte Carlo methods to capture the statistical behaviour of quantum mechanics, it was possible to extract the temporal dynamics of hydrogen and once ionized helium, and get good agreement with recent articles published by [Nagele et al. 2011] (Journal of Physics B: Atomic, Molecular and Optical Physics, 44), [Klünder et al. 2011] (Physical Review Letters, 106.14) and [Ivanov and Smirnova 2011] (Physical Review Letters, 107.21). The program was implemented on a Graphics Processing Unit, the computational unit of a graphics card, which allows or massive parallelization of computations using inexpensive computer hardware available to normal consumers.Om man fotojoniserar en atom, dvs. sliter loss en elektron genom att belysa atomen, kommer elektronen att skjuta iväg från atomen med en hastighet som beror på ljusets energi. Dock kommer atomkärnan att försöka få elektronen att återvända, jfr gravitationen. Om dessutom ett elektriskt fält, såsom infrarött laserljus verkar på elektronen under jonisationen och efteråt, kommer elektronen rörelse ytterligare förändras. Elektronerna detekteras sedan och genom att mäta deras energi kan man beräkna vilken styrka det infraröda laserljuset hade vid jonisationstillfället (denna teknik kallas streaking). Det visar sig dock att detta värde inte helt stämmer överens med den styrka fältet faktiskt hade vid jonisationstillfället. Snarare passar värdet med fältets styrka vid en tidpunkt strax innan. Denna tidsskillnad beror på två saker: 1) genom att atomkärnan drar i elektronen fördröjs jonisationen och 2) genom att det infraröda fältet är närvarande under jonisationen påverkas elektronens rörelse i förhållande till atomkärnan på ett komplext sätt. Dessa tidsaspekter på fotojonisationen är mycket intressanta att studera för grundläggande forskning i atomfysik. Om det infraröda laserfältet är mycket starkt, kan det tvinga tillbaka elektronen till atomkärnan, förutsatt att det är riktat åt rätt håll under tillräckligt lång tid. När elektronen närmar sig kärnan, kan den förra spridas mot den senare, "studsa", och röra sig bort med en högre hastighet än den med vilken den närmade sig. Något liknande inträffar när en komet närmar solen och slungas iväg när den passerat. I mitt projekt har jag tittat närmare på dessa fenomen med hjälp av klassiska (i motsats till kvantmekaniska, som annars är vanligast i atomfysiken), statistiska beräkningar av ett slag som kallas Monte Carlo-metoder (namnet kommer från statistiska studier av tärningskastande som ju inte är en obekant företeelse på kasinot i denna stad). Dessa har utförts på ett grafikkort i en vanlig dator. Sådana grafikkort är speciellt lämpade för sådana här statistiska beräkningar, där många oberoende försök måste göras för att ett tillförlitligt resultat skall uppnås

Phase metrology with multi-cycle two-colour pulses

Strong-field phenomena driven by an intense infrared (IR) laser depend on during what part of the field cycle they are initiated. By changing the sub-cycle character of the laser electric field it is possible to control such phenomena. For long pulses, sub-cycle shaping of the field can be done by adding a relatively weak, second harmonic of the driving field to the pulse. Through constructive and destructive interference, the combination of strong and weak fields can be used to change the probability of a strong-field process being initiated at any given part of the cycle. In order to control sub-cycle phenomena with optimal accuracy, it is necessary to know the phase difference of the strong and the weak fields precisely. If the weaker field is an even harmonic of the driving field, electrons ionized by the field will be asymmetrically distributed between the positive and negative directions of the combined fields. Information about the asymmetry can yield information about the phase difference. A technique to measure asymmetry for few-cycle pulses, called Stereo-ATI (Above Threshold Ionization), has been developed by [Paulus G G, et al 2003 Phys. Rev. Lett. 91]. This paper outlines an extension of this method to measure the phase difference between a strong IR and its second harmonic

General Time-Dependent Configuration-Interaction Singles I: The Molecular Case

We present a grid-based implementation of the time-dependent configuration-interaction singles method suitable for computing the strong-field ionization of small gas-phase molecules. After outlining the general equations of motion used in our treatment of this method, we present example calculations of strong-field ionization of He, LiH, H2O, and C2H4 that demonstrate the utility of our implementation. The following paper [S. Carlström et al., following paper, Phys. Rev. A 106, 042806 (2022)] specializes to the case of spherical symmetry, which is applied to various atoms

General Time-Dependent Configuration-Interaction Singles I: The Molecular Case

We present a grid-based implementation of the time-dependent configuration-interaction singles method suitable for computing the strong-field ionization of small gas-phase molecules. After outlining the general equations of motion used in our treatment of this method, we present example calculations of strong-field ionization of helium, lithium hydride, water, and ethylene that demonstrate the utility of our implementation. The following companion paper specializes to the case of spherical symmetry, which is applied to various atoms

Frustrated tunneling dynamics in ultrashort laser pulses

We study a model for frustrated tunneling ionization using ultrashort laser pulses. The model is based on the strong field approximation and it employs the saddle point approximation to predict quasiclassical trajectories that are captured on Rydberg states. We present a classification of the saddle-point solutions and explore their behavior as functions of angular momentum of the final state, as well as the carrier--envelope phase (CEP) of the laser pulse. We compare the final state population computed by the model to results obtained by numerical propagation of the time-dependent Schr\"odinger equation (TDSE) for the hydrogen atom. While we find qualitative agreement in the CEP dependence of the populations in principal quantum numbers, $n$, the populations to individual angular momentum channels, $\ell$, are found to be inconsistent between model and TDSE. Thus, our results show that improvements of the quasiclassical trajectories are in order for a quantitative model of frustrated tunneling ionizaiton

High-order harmonic generation using a high-repetition-rate turnkey laser

We generate high-order harmonics at high pulse repetition rates using a turnkey laser. High-order harmonics at 400 kHz are observed when argon is used as target gas. In neon we achieve generation of photons with energies exceeding 90 eV ($\sim$13 nm) at 20 kHz. We measure a photon flux of 4.4$\cdot10^{10}$ photons per second per harmonic in argon at 100 kHz. Many experiments employing high-order harmonics would benefit from higher repetition rates, and the user-friendly operation opens up for applications of coherent extreme ultra-violet pulses in new research areas

General Time-Dependent Configuration-Interaction Singles II: The Atomic Case

We present a specialization of the grid-based implementation of the time-dependent configuration-interaction singles described in the preceding paper [S. Carlström et al., preceding paper, Phys. Rev. A 106, 043104 (2022)]. to the case of spherical symmetry. We describe the intricate time propagator in detail and conclude with a few example calculations. Among these, of note are high-resolution photoelectron spectra in the vicinity of the Fano resonances in photoionization of neon and spin-polarized photoelectrons from xenon, in agreement with recent experiments

Control of Spin Polarization through Recollisions

Using only linearly polarized light, we study the possibility of generating spin-polarized photoelectrons from xenon atoms. No net spin polarization is possible, since the xenon ground state is spin-less, but when the photoelectron are measured in coincidence with the residual ion, spin polarization emerges. Furthermore, we show that ultrafast dynamics of the recolliding photoelectrons contribute to an apparent flipping of the spin of the photoelectron, a process that has been completely neglected so far in all analyses of recollision-based processes. We link this phenomenon to the ``spin--orbit clock'' of the remaining ion. These effects arise already in dipole approximation