Abstract Coherent control calculations are presented for a spherically symmetric box potential for non-resonant two photon transition probabilities. With the help of a genetic algorithm (GA), the population of the excited states are maximized and minimized. The external driving field is a superposition of three intensive extreme ultraviolet (XUV) linearly polarized laser pulses with different frequencies in the femtosecond duration range. We solved the quantum mechanical problem within the dipole approximation. Our investigation clearly shows that the dynamics of the electron current has a strong correlation with the optimized and neutralizing pulse shape.
Abstract Coherent control calculations are presented for a spherically symmetric box potential for non-resonant two photon transition probabilities. With the help of a genetic algorithm (GA), the population of the excited states are maximized and minimized. The external driving field is a superposition of three intensive extreme ultraviolet (XUV) linearly polarized laser pulses with different frequencies in the femtosecond duration range. We solved the quantum mechanical problem within the dipole approximation. Our investigation clearly shows that the dynamics of the electron current has a strong correlation with the optimized and neutralizing pulse shape.
Extreme ultraviolet (XUV) light is a valuable frequency range for ultrafast optics experiments. The short wavelength is a requirement for ultrashort pulses, which are widely used to characterize fast dynamics. Additionally, the high photon energy enables quantum control of high frequency transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Presently, however, no good modulators exist for these frequencies, which significantly reduces the possible ultrafast optics experiments or applications in the XUV regime. This thesis addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light a method is described which modulates the phase of XUV light emitted from a gas of noble atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation. After the excitation pulse has passed through the gas, the atoms will emit light in the absence of an external field, known as free induction. The phase of the emitted light is changed by shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse. This phase-control of the emitting atoms also translates into a control of the phase of the emitted XUV field.With the method described in the previous paragraph, the direction of XUV light emitted from noble gases was controlled, and the temporal dynamics of the emission was studied. The redirection of the XUV emission, through the use of a spatially offset IR pulse, was found to change both with intensity of the control pulse and by changing the spatial offset between the pulses. By varying the delay between the excitation and the control pulse the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, with two control pulses the emission was redirected twice, resulting in an XUV pulse shape with controlled duration emitted with a certain angle. Hopefully the presented work will result in more research into the development of this technique, to fully realize the possibility to shape the amplitude and phase of XUV pulses. Such a development would open the door for ultrafast XUV quantum control.
Atoms irradiated with combined femtosecond laser and extreme ultraviolet (XUV) fields ionize through multiphoton processes, even when the energy of the XUV photon is below the ionization potential. However, in the presence of two different XUV photons and an intense laser field, it is possible to induce full electromagnetic transparency. Taking helium as an example, the laser field modifies its electronic structure, while the presence of two different XUV photons and the laser field leads to two distinct ionization pathways that can interfere destructively. This work demonstrates a new approach for coherent control in a regime of highly excited states and strong optical fields.
We report on numerical results revealing line-shape asymmetry changes of electronic transitions in atoms near-resonantly driven by intense extreme-ultraviolet (XUV) electric fields by monitoring their transient absorption spectrum after transmission through a moderately dense atomic medium. Our numerical model utilizes ultrashort broadband XUV laser pulses varied in their intensity (1014–1015 W/cm2) and detuning nearly out of resonance for a quantitative evaluation of the absorption line-shape asymmetry. It will be shown how transient energy shifts of the bound electronic states can be linked to these asymmetry changes in the case of an ultrashort XUV driving pulse temporally shorter than the lifetime of the resonant excitation, and how the asymmetry can be controlled by the near-resonant detuning of the XUV pulse. In the case of a two-level system, the numerical model is compared to an analytical calculation, which helps to uncover the underlying mechanism for the detuning- and intensity-induced line-shape modification and links it to the generalized Rabi frequency. To further apply the numerical model to recent experimental results of the near-resonant dressing of the 2s2p doubly excited state in helium by an ultrashort XUV free-electron laser pulse we extend the two-level model with an ionization continuum, thereby enabling the description of transmission-type (Fraunhofer-like) transient absorption of a strongly laser-coupled autoionizing state.
The amplification of ultrashort extreme-ultraviolet (XUV) pulses in argon in high-order harmonic generation processes is studied by using the time-dependent Schr\"odinger equation in the spin-free one-active-electron and single-atom approximation. We consider a neutral argon atom initially in the valence 3p state and a sufficiently intense two-cycle driving infrared (IR) pulse for the atom to be mainly ionized after the first laser cycle. The correlated dynamics and transitions from the valence 3p to a virtual subvalence 3s state and in the ionized regions are examined by synchronizing a 1.5-fs XUV pulse to the IR pulse. The calculated single-atom gain spectrum (26--45 eV) agrees with recent experimental measurements. We discuss different channels that can be present in the gain process as a function of pulse parameters and through an analysis of the dynamics of the populations in terms of field-free eigenstates. When the XUV pulse is considered at the end of the driving IR field the amplification is due to contributions of stimulated recombination from excited Rydberg and low energy continuum states to the 3s and 3p states of argon. In regions where the IR field is intense, high energy and angular momentum states are populated and the medium can interact with the pulses through bound and continuum states involving parametric transitions, which is further confirmed by studying classical electron trajectories. We discuss how these parametric interactions might be suitable for amplification of photon energies far from the ionization threshold as observed in the experiments.
Abstract Ultrashort, coherent light pulses in the extreme ultraviolet (XUV) wavelength region are excellent tools for a wide range of experiments. These XUV pulses are in particular used to characterize electron dynamics. Additionally, the high photon energy enables quantum control of short-lived transitions in atoms. These transitions are of interest for ultrafast quantum control as the bandwidth of the control pulses needs to be less than the transition frequency. Coherent XUV pulses would be even more useful if the tools to control and manipulate them existed. Presently, however, no modulators exist for these frequencies, which significantly reduces the number of possible ultrafast optics experiments and applications in the XUV regime. This Ph.D. tutorial addresses this problem and focuses on the control of XUV light in time and space. To enable control of XUV light we have developed a method for opto-optical modulation (OOM), which modulates the phase of XUV light emitted from a gas of atoms. The atoms are resonantly excited with a coherent XUV pulse generated through high order harmonic generation and emit light after the excitation pulse passed. By shifting the resonance frequencies through the AC Stark shift with a non-resonant infrared control pulse, the phase of the emitted light, and therefore its wavefront, is controlled. With OOM the direction of XUV light emitted from noble gases has been controlled, and the temporal dynamics of the emission studied. By varying the delay between the excitation and the control pulse, the time of redirection was controlled, which enabled high signal to noise study of the temporal dynamics of the emission. Furthermore, a proof-of-principle experiment with two control pulses was performed, resulting in a shaped XUV pulse with controlled duration. The technique of OOM presented in this article is still at a very early stage and in order to obtain complete control of the amplitude and phase of the XUV pulses further development is necessary. Such development would present new opportunities for ultrafast XUV quantum control.
Abstract Coherent control uses interference between quantum pathways to enhance or suppress transitions to a target quantum state. In stimulated Raman excitation, such control can be achieved through interference between two indistinguishable pathways connecting the same initial and final states, with the relative phase between the pathways modulating the population transfer to the final state. Extending this strategy to the extreme ultraviolet (XUV) and X-ray regimes would enable control of ultrafast electronic dynamics and site-selective core excitations. However, Raman coupling at short wavelengths competes with photoionization and intermediate-state decay, whereas intense fields with stable and adjustable relative phases remain difficult to produce. Here we use two-colour pulses from a seeded coherent free-electron laser to excite helium from its ground state to a bound metastable state via stimulated XUV Raman excitation. We implement phase-coherent control by creating two interfering Raman pathways through the ionization continuum and modulate the transferred population by varying their relative phases. Blocking one pathway removes the modulation, confirming its interferometric origin, and time-dependent Schrödinger equation calculations reproduce the measured phase dependence. Our results establish phase-controlled stimulated Raman excitation in the XUV regime, providing a strategy for the coherent control of atomic and molecular processes at short wavelengths.