We report velocity map imaging experiments on the behaviour of electrons that are strongly driven by electric fields that vary over a wide range of timescales (from DC electric fields to laser fields at optical frequencies). We show how in slow photoelectron imaging using DC electric fields direct and indirect photoionization events can be distinguished and that a quantummechanical interference between several competing pathways that take the electron from the atom to the detector can be observed. We show that with a half-cycle THz pulse Rydberg states can be ionized, that the electrons can be accelerated (though the final kinetic energy of the electrons is small) and that the initial momentum distribution of the prepared Rydberg state can be observed. Finally, we show that—at optical frequencies—high harmonic generation can lead to the formation of trains of attosecond laser pulses, with a striking dependence of the appearance time of the attosecond pulses on the harmonic generation gas.
We report velocity map imaging experiments on the behaviour of electrons that are strongly driven by electric fields that vary over a wide range of timescales (from DC electric fields to laser fields at optical frequencies). We show how in slow photoelectron imaging using DC electric fields direct and indirect photoionization events can be distinguished and that a quantummechanical interference between several competing pathways that take the electron from the atom to the detector can be observed. We show that with a half-cycle THz pulse Rydberg states can be ionized, that the electrons can be accelerated (though the final kinetic energy of the electrons is small) and that the initial momentum distribution of the prepared Rydberg state can be observed. Finally, we show that—at optical frequencies—high harmonic generation can lead to the formation of trains of attosecond laser pulses, with a striking dependence of the appearance time of the attosecond pulses on the harmonic generation gas.
Using high-order harmonic attosecond pulse trains, we investigate the photoionization dynamics and transient electronic structure of a helium atom in the presence of moderately strong (∼10(12) W cm(-2)) femtosecond laser pulses. We observe quantum interferences between photoexcitation paths from the ground state to different laser-dressed Floquet state components. As the intensity ramps on femtosecond time scales, we observe switching between ionization channels mediated by different atomic resonances. Using precision measurements of ion yields and photoelectron distributions, the quantum phase difference between interfering paths is extracted for each ionization channel and compared with simulations. Our results elucidate photoionization mechanisms in strong fields and open the doors for photoabsorption or photoionization control schemes.
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 asymptotic velocity distribution of electrons ionized in half-cycle-pulse excitation of high Rydberg states (n=34), placed in a static electric field, is studied using electron velocity-map imaging. At weak half-cycle pulse strengths, the electrons escape over the saddle point in the potential. For strong half-cycle pulses, the electrons are emitted in the direction of the field kick. The much slower and less intense half cycle of opposite polarity, which necessarily follows the main half-cycle pulse, strongly affects the momentum distribution and reduces the excess energy of the electrons significantly.
To time-resolve attosecond electronic dynamics in general photoionization processes, the technique that retrieves the phase of emitted electronic wave packets without intercepting the interactions is essential. Here, we theoretically demonstrate a scheme that uses isolated attosecond pulses (IAPs) to achieve this goal. Our approach utilizes the coherent interference between the electronic wave packets of interest and the one produced by a subsequent IAP. It is shown that the photoelectron spectral phase that has eluded direct detection so far can be fully recovered from observable photoelectron spectra without perturbing the electron-release process under investigation. By further performing a time-frequency-like analysis on the photoelectron energy spectra with the spectral phase, we reveal the birth processes of photoelectrons in time and the association between electronic energy and birth time in strong-field ionization driven by circularly polarized laser pulses. The present work explores a promising application of IAPs for ultrafast measurement and opens a viable venue for investigating electronic dynamics with quantum phase information.
Abstract In atomic strong field ionization, a photoelectron’s motion is largely determined by the laser pulse’s expected electric field, as well as the atomic potential. However, when the ionization is performed by squeezed light, the photon’s quantum statistics can modify these dynamics. In this work, we generalize the classical trajectory Monte–Carlo technique to study strong field ionization by squeezed states of light. We find that electrons driven by phase-squeezed pulses exhibit increases in longitudinal momentum consistent with what is expected from the field statistics. Meanwhile, the transverse momentum distribution shows varying influence of ‘hard’ and ‘soft’ rescattering off the Coulomb potential, and the proportion of electrons recaptured as Rydberg states decreases monotonically with the amount of squeezed light applied. This work demonstrates the added experimental control over ultrafast electron dynamics achievable by tuning light’s quantum statistics, as well as the essential role of the atomic potential in strong field ionization.
Photoionization is one of the most fundamental photoreactions that leads to the ejection of an electron and the formation of an ion. With the help of modern 3D momentum imaging spectrometers, the interplay between the time and frequency domain upon ionization is studied. In this work, single photoionization of helium is investigated in the few-cycle pulse regime of the driving infrared laser pulses with ultrashort attosecond pulses in a dressing infrared field. By varying the carrier-to-envelope phase of the infrared field to modify the spectro-temporal structure of the attosecond pulses, we analyze how the infrared field impacts the measured angularly-resolved photoelectron energy distribution. To this end, a newly built coincidence 3D momentum spectrometer was used. A thorough understanding of the experiment is achieved by using simulations within the strong field approximation. This analysis allows us to separate our observation into two scenarios, one at two attosecond pulses when the carrier-to-envelope phase is π/2 and the second one at three attosecond pulses when the carrier-to-envelope phase is 0 or π. When the dressing infrared field is added, the photoelectron spectrum changes in comparison to photoionization with attosecond pulses only. While two attosecond pulses in the dressing infrared field modify the photoelectron spectrum in a continuous way, we observe sidebands appearing in the spectrum generated by three pulses in the dressing infrared field. Finally, the resolution of the spectrometer is studied before and after an upgrade of the spectrometer was performed in the context of this work. This study allows us to understand the limiting factors of the energy resolution. In particular, it shows that the resolution is essentially limited by the source volume of the emitted particles, while the magnetic field does not limit the resolution as it was expected.