We demonstrated coherent control of molecular absorption line shape and optical gain in XUV. The control is achieved by creating a quantum coherence in the ground electronic state of hydrogen molecules. These observations provide new insights into control of spectral line shapes and open the way for achieving useful laser pulses in the XUV spectral range.
We demonstrated coherent control of molecular absorption line shape and optical gain in XUV. The control is achieved by creating a quantum coherence in the ground electronic state of hydrogen molecules. These observations provide new insights into control of spectral line shapes and open the way for achieving useful laser pulses in the XUV spectral range.
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.
We employ an extreme ultraviolet (XUV) pulse to impulsively excite dipole polarization in atoms or molecules, which corresponds to coherently prepared superposition of excited states. A delayed near infrared (NIR) pulse then perturbs the fast evolving polarization, and the resultant absorbance change is monitored in dilute helium, dense helium, and sulfur hexafluoride (SF6) molecules. We observe and quantify the time-dependence of various transient phenomena in helium atoms,includinglaser-inducedphase(LIP),time-varying(AC)Starkshift,quantumpathinterference, and laser-induced continuum structure. In the case of dense helium targets, we discuss nonlinear macroscopic propagation effects pertaining to LIP and resonant pulse propagation, which accoun tfor the appearance of new spectral features in transient lineshapes. We then use tunable NIR photons to demonstrate the wavelength dependence of the transient laser induced effects. In the case of molecular polarization experiment in SF6, we show suppression of XUV photoabsorption corresponding to inter-valence transitions in the presence of a strong NIR field. In each case, the temporal evolution of transient absorption spectra allows us to observe and understand the transient laser induced modifications of the electronic structure of atoms and molecules.
Ultrafast laser pulses with broad bandwidth can simultaneously excite multiple quantum states, forming coherent superpositions. The phases of the associated optical transitions evolve in time, giving rise to spectral line profiles that range from symmetric Lorentzian to asymmetric Fano-like forms. Yet, the line shape relations between different transitions within such superpositions remain largely unexplored. Here, we use transient absorption spectroscopy to probe transitions from coherent superpositions. While each transition’s line profile evolves over time, the relative line shape relations between different transitions remain fixed. We identify 4 fundamental types of line shape relations, each producing distinct spectral signatures. These relations naturally arise from second-order perturbation processes and can be actively tuned via the laser–matter interaction strength. Our findings establish a pathway for line shape-engineered control across atomic and molecular systems.
We investigate the evolution of extreme ultraviolet (XUV) spectral line shapes in an optically thick helium gas under near-infrared (IR) perturbation. In our experimental and theoretical work, we systematically vary the IR intensity, time-delay, gas density, and IR polarization parameters to study line-shape modifications induced by collective interactions in a regime beyond the single-atom response of a thin, dilute gas. In both experiment and theory, we find that specific features in the frequency-domain absorption profile, and their evolution with propagation distance, can be attributed to the interplay between resonant attosecond pulse propagation and IR-induced phase shifts. Our calculations show that this interplay also manifests itself in the time domain, with the IR pulse influencing the reshaping of the XUV pulse propagating in the resonant medium.
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.