Coherent and statistical phase control and measurements of time-dependent quantum dynamics

In this work the importance of phases in time-resolved spectroscopy is investigated in two respects. At first, the influence of the phase of a system’s dipole response after excitation is studied. Previous transient-absorption experiments in helium allowed the measurement and control of this phase of an emitting dipole by a second, coupling laser pulse. The derived concept is now generalized to a more complex system, i.e. a dye molecule in the liquid phase. For this purpose, a setup for transient-absorption measurements with femtosecond infrared laser pulses is developed and assembled and numerical simulations support the interpretation of the experimental results. It was found that only specific excited states couple strongly to the laser field. While the foregoing experiments rely on the full coherence of laser pulses, the second part of this work addresses the impact of partially coherent phases of laser pulses. Pump–probe experiments in gaseous deuterium molecules applied statistically fluctuating pulses delivered by a Free-Electron Laser source. These measurements revealed an enhanced temporal resolution on time scales shorter than the average pulse duration. For the description and explanation of the observations a novel approach is developed which is based on the correlation of temporally random substructures of the pulses. In order to realize noisy pulses in the laboratory, a pulse shaper is designed and built up which is capable to modify the spectral phase of the laser pulses. Thereby, this developed general method is transferred to transient-absorption measurements in the liquid phase and its universal applicability is demonstrated

Attosecond interferometry: techniques and spectroscopy

The interaction between an intense laser pulse and a gas medium leads to the emission of coherent bursts of light in the extreme ultraviolet range. This process, known as high-order harmonic generation, has today, almost three decades after its discovery, developed into a reliable source of extremely short (on the order of 100 as) pulses of electromagnetic radiation, with a wide range of applications in the atomic, molecular and optical sciences. The access to radiation with attosecond duration opens up new possibilities for studying and even controlling electronic processes that take place on this timescale. This thesis presents a series of experiments where sequences of attosecond pulses, attosecond pulse trains, are used to perform photoelectron interferometry. Free electronic wave packets, launched via photoabsorption of a coherent train of ionizing attosecond pulses, are manipulated by an infrared laser field and brought to interfere. From the resulting interferogram the phase of the escaping wave packets can be partly reconstructed. This phase in turn carries a signature of the interactions that lead to the ejection of the electron. Under certain conditions the measured phase can be related to a delay of the wave packet, corresponding to the time it takes for it to escape the ionic binding potential, called photoionization time delay. This method was applied to a range of atomic systems and ejection mechanisms in order to study the influence of atomic electronic structure on the ejection of electrons. Since the composition of the electronic wave packets is partly determined by the temporal structure of the ionizing radiation, a comparative approach was applied to isolate the effect of the ion-electron interactions. The photoionization time delay for ionization from the 3s subshell of argon was measured relative to that of the 3p shell. In another experiment the delay of a two electron wave packet resulting from double ionization of xenon was referenced to single ionization from the valence shell. In an iterative measurement procedure, interferograms were cross-referenced from ionization of the valence shells of argon, helium and neon. Finally, the significant phase distortion resulting from an autoionizing resonance in argon was mapped out by stepwise tuning the central frequency of the exciting pulse train. The interferometric method was also utilized to study the temporal synchronization between the attosecond pulse train and the laser pulse used to produce it. The results show that the synchronization is dependent on the density of the gaseous medium due to the specific dispersion properties of the gas

Attosecond physics at the nanoscale

Recently two emerging areas of research, attosecond and nanoscale physics, have started to come together. Attosecond physics deals with phenomena occurring when ultrashort laser pulses, with duration on the femto- and sub-femtosecond time scales, interact with atoms, molecules or solids. The laser-induced electron dynamics occurs natively on a timescale down to a few hundred or even tens of attoseconds, which is comparable with the optical field. On the other hand, the second branch involves the manipulation and engineering of mesoscopic systems, such as solids, metals and dielectrics, with nanometric precision. Although nano-engineering is a vast and well-established research field on its own, the merger with intense laser physics is relatively recent. In this article we present a comprehensive experimental and theoretical overview of physics that takes place when short and intense laser pulses interact with nanosystems, such as metallic and dielectric nanostructures. In particular we elucidate how the spatially inhomogeneous laser induced fields at a nanometer scale modify the laser-driven electron dynamics. Consequently, this has important impact on pivotal processes such as ATI and HHG. The deep understanding of the coupled dynamics between these spatially inhomogeneous fields and matter configures a promising way to new avenues of research and applications. Thanks to the maturity that attosecond physics has reached, together with the tremendous advance in material engineering and manipulation techniques, the age of atto-nano physics has begun, but it is in the initial stage. We present thus some of the open questions, challenges and prospects for experimental confirmation of theoretical predictions, as well as experiments aimed at characterizing the induced fields and the unique electron dynamics initiated by them with high temporal and spatial resolution

Coherent and statistical phase control and measurements of time-dependent quantum dynamics

In this work the importance of phases in time-resolved spectroscopy is investigated in two respects. At first, the influence of the phase of a system’s dipole response after excitation is studied. Previous transient-absorption experiments in helium allowed the measurement and control of this phase of an emitting dipole by a second, coupling laser pulse. The derived concept is now generalized to a more complex system, i.e. a dye molecule in the liquid phase. For this purpose, a setup for transient-absorption measurements with femtosecond infrared laser pulses is developed and assembled and numerical simulations support the interpretation of the experimental results. It was found that only specific excited states couple strongly to the laser field. While the foregoing experiments rely on the full coherence of laser pulses, the second part of this work addresses the impact of partially coherent phases of laser pulses. Pump–probe experiments in gaseous deuterium molecules applied statistically fluctuating pulses delivered by a Free-Electron Laser source. These measurements revealed an enhanced temporal resolution on time scales shorter than the average pulse duration. For the description and explanation of the observations a novel approach is developed which is based on the correlation of temporally random substructures of the pulses. In order to realize noisy pulses in the laboratory, a pulse shaper is designed and built up which is capable to modify the spectral phase of the laser pulses. Thereby, this developed general method is transferred to transient-absorption measurements in the liquid phase and its universal applicability is demonstrated. _________________________________________ Readers may view, browse, and/or download material for temporary copying purposes only, provided these uses are for noncommercial personal purposes. Except as provided by law, this material may not be further reproduced, distributed, transmitted, modified, adapted, performed, displayed, published, or sold in whole or part, without prior written permission from the American Physical Society

26th Symposium on Plasma Physics and Technology

List of abstract

Magnetic micro-confinement of quantum degenerate gases

In this dissertation we explore the basic principles of the magnetic micro-confinement of the quantum degenerate gases where the approach of the so-called two-dimensional magnetic lattices has been theoretically and experimentally investigated. In this research a new generation of two-dimensional magnetic lattice has been proposed and considered as a developing phase for the previous approaches. Its advantage relies on introducing a simplified method to create single or multiple micro-traps of magnetic field local minima distributed, at a certain working distance, above the surface of a thin film of permanent magnetic material. The simplicity in creating the magnetic field local minima at the micro-scale manifests itself as a result of imprinting specific patterns through the thin film using suitable and available micro-fabrication techniques. In this approach, to create multiple micro-traps, patterned square holes of size αh X αh spaced by αs are periodically distributed across the x/y plane taking a two-dimensional grid configuration. These magnetic field local minima are recognized by their ability to trap and confine quantum single-particles and quantum degenerate gases at various levels of distribution in their phase spaces, such as ultracold atoms and virtual quantum particles. Based on the nature of the interaction between the external confining potential fields and the different types of quantum particles, this research is conducted through two separate but not different phases. We performed theoretical and/or experimental investigations, for both phases, at the vicinity of the magnetic micro-confinement and its suitability for trapping quantum particles. A special attention is paid to inspect the coherence in such systems defined in terms of providing an accessible coupling to the internal quantum states of the magnetically trapped particles. Such coherence is considered as one of the important ingredients for simulating condensed matter systems and processing quantum information