Recent advances in the ultrashort laser technology have spawned a series of new directions of research in atomic and molecular physics. This work compounds two studies on the control and probing of atomic matter by ultrafast laser pulses. The first study is concerned with the control of quantum wavepackets focusing by chirped, ultrashort laser pulses. The ability to control wavepacket focusing is important for the achievement of specific excitation pathways, for controlling chemical reactivity or even molecular synthesis. In this work, we present a general method for wavepacket control in the weak field limit. Numerical simulations involving the ICN molecule confirm our theoretical framework. The second study refers to the ionization of sodium Rydberg states via half-cycle pulses (HCPs). For this process, our full quantum-mechanical calculations demonstrate an asymmetry in both ionization rates and ionization spectrum between the downhill (Stark red-shifted) and uphill (Stark blue- shifted) states. The asymmetry is explained by appealing to a semi-classical model of the interfering ionization pathways.
Recent advances in the ultrashort laser technology have spawned a series of new directions of research in atomic and molecular physics. This work compounds two studies on the control and probing of atomic matter by ultrafast laser pulses. The first study is concerned with the control of quantum wavepackets focusing by chirped, ultrashort laser pulses. The ability to control wavepacket focusing is important for the achievement of specific excitation pathways, for controlling chemical reactivity or even molecular synthesis. In this work, we present a general method for wavepacket control in the weak field limit. Numerical simulations involving the ICN molecule confirm our theoretical framework. The second study refers to the ionization of sodium Rydberg states via half-cycle pulses (HCPs). For this process, our full quantum-mechanical calculations demonstrate an asymmetry in both ionization rates and ionization spectrum between the downhill (Stark red-shifted) and uphill (Stark blue- shifted) states. The asymmetry is explained by appealing to a semi-classical model of the interfering ionization pathways.
Efficient excitation of an ensemble of atoms and molecules to a desired state is of fundamental importance not only for spectroscopy but also for quantum control of molecular dynamics in chemical and biological processes. The latter usually occurs on a much shorter time scale (in the femtosecond range) giving rise to steadily advanced problem how to excite one of the nearby levels, even when they are well within the broad fs pulse spectrum. We propose a new effective method for selective excitation of atoms based on the atomic coherence and discuss a sensitive method to verify experimentally the proposed mechanism using the technique of quantum beating in pump-probe experiments.
We demonstrate that atomic wavepackets can serve as sensitive detectors for investigating atomic and molecular dynamics. In concert with parametric four-wave mixing, the interference between coherent superpositions of atomic excited states produced by ultrafast (/spl sim/150 fs) pump and probe pulses provides a new and powerful tool with which fundamental processes, such as molecular dissociation and Rydberg-Rydberg atomic collisions, can be observed with the extraordinary sensitivity afforded by a coherent nonlinear optical process. Experiments are described in which the dissociation of an electronically excited molecule (Rb/sub 2/) and the distribution of atomic fragments into excited states spanning >10 000 cm/sup -1/ are observed. Also, resonant collisions between Rb atoms in the 7s and 5d states are detected by monitoring the shift in the frequency of an atomic wavepacket induced by the dipole-dipole interaction.
The past thirty years witnessed rapid developing of strong field physics that leads to the first generation of attosecond (10−18 s) pulses. The duration of available pulses has recently been reduced to less than 100 as, approaching the Kepler period of a classical electron revolving around the nucleus. For the first time, electron motions inside atoms, molecules and solids can be traced, probed and even steered in real time. It pushed the study on ultrafast dynamic processes into a new frontier, giving birth to attosecond physics. The development of attosecond pulses provides unprecedented coherent ultrashort XUV (extreme ultra-violet) of soft X-ray sources that enable imaging the atomic structure and resolving the related quantum dynamics for matter science. In particular, it allows the probing and monitoring of electron dynamics in the natural time scale of attoseconds with atomic spatial resolution. The generation and applications of attosecond pulses are directly related to the control and manipulation of the sub-cycle electron dynamics. Recent studies on the correlated multielectron dynamics have provided insight into the optical and dynamical properties of matter in the perspectives of time, phase and entanglement. It deepens our understanding on some of the fundamental questions, e.g., how the single or multiple photons are absorbed and how short are the quantum processes such as photoionization, tunneling ionization and charge migration. Although currently attosecond pulses are limited by the flux and pulse duration, the combination of attosecond pulses with intense infrared laser pulses paves new ways of controlling and probing electron dynamics in the domains of energy, time and space that lays the foundation for petahertz optoelectronics and attosecond pulse based transient spectroscopy. In this review, we will first briefly summarize the advancement of strong field and ultrafast physics including the relevant experimental findings, the theoretical explorations, the related technologies and the possible applications. Then we focus on the sub-cycle electron or multielectron dynamics and their manifestation on the ionization and coherent radiation properties. We start with the time behavior of Fano resonance and how it can be probed in time domain. In section 2, the correlated electron dynamics is discussed in term of anti-screening effect and Pauli effect that influence strong field ionization and high harmonic generation processes. In section 3, the coherent emission with frequency from terahertz to soft XUV and their joint measurement are discussed by emphasizing the generation mechanism of terahertz wave in two-color laser pulses. The developed high-harmonic and terahertz wave spectroscopy (HATS) is shown capable of imaging the molecular structure and dynamics. In section 4, the time-resolving of electron and hole dynamics and their coherence is proposed based on attosecond transient absorption technique. The coherence of ionization and the correlated electron dynamics is hinted crucial for electron-hole interaction. The topics are very limited and mainly selected from the perspectives of our group.
The main goal of this PhD work is an experimental study of coherent excitation of atomic and molecular wavepackets, i.e. superpositions of many quantum eigenstates, by shaped femtosecond pulses. Approaches allowing nearly complete population transfer between quantum eigenstates were well studied in the past within the two level approximation. In this work we focus on adiabatic and non-adiabatic methods of population transfer beyond the two-level approximation. Excitation of multi-level target states is possible due to broad spectrum of an ultrashort pulse which contains frequencies needed for multiple transitions to different states in the final superposition. At the same time, the spectrum of an ultrashort pulse can be modified, or ``shaped'', in order to affect the excitation process and control the amplitudes in the final superposition. Both non-adiabatic and quasi-adiabatic methods were first implemented and studied in electronic wavepackets in alkali atoms. The non-adiabatic approach revealed features linked to the strong-field perturbations of the energy level structure of the quantum system. An adiabatic method was implemented for the first time on a femtosecond time scale, and was thoroughly characterized. The control over complex amplitudes in the target superposition was demonstrated as well as completeness of the population transfer. In the second part of this work, we focused on coherent control of rotational wavepackets in diatomic molecules. Rotational excitation by a periodic train of femtosecond pulses was investigated in the context of ``delta-kicked'' rotor - a paradigm system for studying quantum chaos, and the effect of quantum resonance was demonstrated for the first time in a system of true quantum rotors. Control of uni-directional molecular rotation was proposed and demonstrated with a novel ``chiral pulse train'' - a sequence of femtosecond pulses with polarization rotating from pulse to pulse by a predefined angle. All the developed techniques offer new tools in coherent control of atomic and molecular wavepackets on an ultrashort time scale.
Quantum excitation is usually regarded as a transient process occurring instantaneously, leaving the underlying physics shrouded in mystery. Recent research shows that Rydberg-state excitation with ultrashort laser pulses can be investigated and manipulated with state-of-the-art few-cycle pulses. We theoretically find that the efficiency of Rydberg state excitation can be enhanced with a short laser pulse and modulated by varying the laser intensities. We also uncover new facets of the excitation dynamics, including the launching of an electron wave packet through strong-field ionization, the re-entry of the electron into the atomic potential and the crucial step where the electron makes a U-turn, resulting in twin captures into Rydberg orbitals. By tuning the laser intensity, we show that the excitation of the Rydberg state can be coherently controlled on a sub-optical-cycle timescale. Our work paves the way toward ultrafast control and coherent manipulation of Rydberg states, thus benefiting Rydberg-state-based quantum technology.
This thesis presents original work contributing to the understanding of coherent control of wave packet dynamics in atomic and molecular systems excited by sequences of ultrafast laser pulses. An introduction to coherent control and a review of the literature is presented in chapter 1. Different approaches to coherent control are considered from the perspective of the phase complexity of the light fields used to achieve them, starting with a single transform-limited pulse, advancing through two-colour and multi-pulse schemes to chirped pulses, and finally arbitrarily shaped pulses. The purpose of this literature review is to position the work described in this thesis within the context of contemporary coherent control. In particular, our approach is seen to be in line with the current trend towards understanding the physics underlying coherent control processes. In chapter 2, development of a sodium atom and dimer beam source for use in optical coherent control experiments is described. The concepts of molecular beams are introduced along with a brief review of the literature on sodium atom and dimer beams. The beam source is considered in two parts. Firstly the sodium oven and nozzle are described in detail, along with the vacuum chamber that houses them. This is followed by a description of the experimental chamber, which contains a time-of-flight mass spectrometer. Finally a number of potential improvements to the system are discussed. Experiments to control the angular momentum composition of Rydberg electronic wave packets in atomic sodium are reported in chapter 3. The control scheme uses phase-locked pairs of transform-limited picosecond laser pulses to create pairs of identical radial wave packets in the Rydberg states. Quantum interference between the wave packets, or the excitation pathways to them, is manipulated to control the total angular composition of the resultant wave packet, which is detected either by the optical Ramsey method or by state-selective field ionisation. The results are interpreted both from the perspective of time evolution of the wave packet and the frequency content of the laser fields. The control strategy is then extended theoretically by the inclusion of linear chirp in one of the laser pulses. Chapter 4 describes progress towards a novel experiment for optical coherent control of vibrational wave packet dynamics in the sodium dimer. A coherent superposition of high-lying Rydberg states of the Na2 is excited by the broad bandwidth of a femtosecond laser pulse. The Rydberg states converge to several vibrational ionisation limits of the dimer, so the excitation effectively forms a vibrational wave packet in the X+ 22g+ ionic potential. Excitation of several wave packets by a series of phase-locked transform-limited pulses allows control over the composition and dynamics of the wave packet, and its detection using a combination of the optical Ramsey method and pulsed-field ionisation techniques adapted from ZEKE spectroscopy. A simple genetic algorithm is presented in chapter 5. It has been implemented to investigate the suitably of this type of optimisation procedure in simulations of Rydberg electronic wave packet coherent control in atomic sodium. The operation of the algorithm is described in detail. It is then tested against known solutions for two experimental scenarios: coherent control based on pairs of transform-limited laser pulses and transform-limited first pulse followed by a chirped second pulse. The behaviour of the algorithm is discussed in detail for each of these cases. Finally, the potential for application of the algorithm to unknown problems is briefly discussed.