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Physical Review A

Strong-field-approximation model for coherent extreme-ultraviolet emission generated through frustrated tunneling ionization

Je Hoi Mun, Igor A. Ivanov, Hyeok Yun, Kyung Taec Kim

Published Dec 27, 2018• 12 citations
Publisher

Abstract

An atom can be excited through tunneling when it is exposed to a strong laser field. This excitation process is known as frustrated tunneling ionization (FTI). Recently it has been reported that coherent extreme-ultraviolet emission can be achieved through FTI. In the present work we extend the strong-field-approximation (SFA) model developed by Lewenstein et al. to derive a quantum mechanical description of the FTI emission. An analytic expression is derived to describe FTI emission as four steps: tunneling of an electron from the ground state of an atom, acceleration of the electron in the continuum, recombination into the exited state of the parent atom, and free induction decay. The coherence property of FTI emission is studied using the attosecond lighthouse method, in which a spatially chirped laser beam generates FTI emissions in different directions. The spatial distribution of the FTI emission in the far field is analyzed using both the extended SFA model and the time-dependent Schr\"odinger equation model. The results obtained from the two models agree well, thereby confirming the validity of our extended SFA model.

DOI
10.1103/physreva.98.063429

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An atom can be excited through tunneling when it is exposed to a strong laser field. This excitation process is known as frustrated tunneling ionization (FTI). Recently it has been reported that coherent extreme-ultraviolet emission can be achieved through FTI. In the present work we extend the strong-field-approximation (SFA) model developed by Lewenstein et al. to derive a quantum mechanical description of the FTI emission. An analytic expression is derived to describe FTI emission as four steps: tunneling of an electron from the ground state of an atom, acceleration of the electron in the continuum, recombination into the exited state of the parent atom, and free induction decay. The coherence property of FTI emission is studied using the attosecond lighthouse method, in which a spatially chirped laser beam generates FTI emissions in different directions. The spatial distribution of the FTI emission in the far field is analyzed using both the extended SFA model and the time-dependent Schr"odinger equation model. The results obtained from the two models agree well, thereby confirming the validity of our extended SFA model.
Substantial neutral atoms can tunnel to excited states in an intense laser field and subsequently generate coherent emission through free induction decay. We experimentally observe an enhanced coherent emission in the harmonic slightly below the threshold, which is consistent with the free induction decay of Rydberg states produced by the frustrated tunnelling ionization (FTI) process. We further find that the intensity of the coherent emission significantly depends on the chirp of laser pulses. The simulations based on the strong field approximation model show that laser chirp affects the probability that the returned electrons recombine to the Rydberg states. Our result shows that coherent emission can be controlled by laser chirp, which facilitates understanding the dynamics of the Rydberg atom and coupling mechanism between the below-threshold harmonics and atomic energy level. In addition, the coherent below-threshold FTI emission we observed has small divergence which is good for EUV light source applications.
Coherent supercontinuum radiation in the extreme-ultraviolet (XUV) range is indispensable for synthesizing attosecond light pulses and for exploring transient atomic structures. Here, we report the striking observations of coherent XUV supercontinuum (XSC) extended from below to far above the ionization threshold, which exhibits completely different temporal and spatial properties comparing to the conventional rescattering induced high harmonic generation (HHG). We demonstrate that the strong-field created coherence among bound orbitals strongly distort the atomic transition energies during the pulse, leading to coherent emission spanning tens of electron-volts, in contrast to the line emission via free-induction decay occurring after the pulse. The supposed non-radiating bound dark states contribute as well by emitting dressed energy through dark-to-bright emission mechanism. All the processes modulated at sub-cycle time scale jointly form this new-type coherent XSC. This work achieves the strong-field attosecond control of the exotic atomic radiation dynamics and provides the means of simultaneous generation of separated attosecond sources, i.e., XSC and HHG, with potential advancing attosecond interferometry.
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.
High-order harmonic generation is a highly nonlinear phenomenon in which ultraviolet photons are generated when an atom is subjected to a high-intensity infrared laser field. This process has been extensively studied, but most of the formulations employ a semiclassical approach in which the electron is treated quantum mechanically while the driving field is treated classically. Although some recent studies follow a quantum-optical approach, the existing descriptions do not offer much insight into the physical processes taking place, and are always limited to monochromatic driving fields. In this thesis, we construct a fully quantized framework for strong-field ionization and high-harmonic generation that allows for quantization of both the driving field and the ultraviolet emission. The presented formalism extends the Strong-Field Approximation theory to a quantum-optical framework, with which we can obtain an analytical expression for the electron and the driving field after the interaction.
A model which describes the time evolution of strong-field photoionization of atoms is presented. Based on the numerical solution of the non-stationary Schrodinger equation, the model allows one to predict and to interpret the results of experiments on double photoionization of atoms by a combined action of a very short (attosecond) XUV pulse and a few-cycle IR pulse of a powerful laser at various delay times between the two pulses. Depending on the binding energy of the ionized electron, two types of processes are considered. If the electron is tightly bound (Ne case), the XUV pulse ionizes the atom and shakes up another (outer) electron to an excited state, which is subsequently ionized by the strong IR field. For an atom with a weakly bound outer electron (Li case), the IR field ionizes the latter, while the XUV pulse, ionizing the inner shell, terminates (or suppresses) the strong-field ionization. In both cases the yield of doubly ionized ions strongly depends on the delay time between the two pulses, revealing 'steps', oscillations and other features which characterize the time evolution of the ionization process. The presented model describes qualitatively the results of recent experiments on Ne.
Abstract We demonstrate that the extreme ultraviolet free induction decay emission can be significantly enhanced by employing isolated attosecond pulses. In this work, the near infrared pulses are applied to excite the neon atoms into Rydberg states coherently, and isolated attosecond pulses are used to manipulate the populations of the Rydberg states and the subsequent free induction decay process. The time resolved experimental measurement of the dependence of the resonance emission yield would help to understand the buildup dynamics of population of excited states. The enhancement assisted by attosecod pulses can serve as a mechanism to develop high-flux extreme ultraviolet light sources.