Strong-field Atomic Physics Research Articles

Low energy structure of above-threshold ionization spectra produced by mid-infrared laser pulses

The low-energy structure (LES) of above-threshold ionization (ATI) of atoms subjected to an intense laser field is a hot topic in the strong-field atomic physics. The rich physical insights behind LES attract a lot of attention. Based on a semi-classical model, a semi-classical two-step (SCTS) quantum trajectory model and numerical solution of the time-dependent Schrödinger equation (TDSE), we study the pulse-duration dependence of LES for Xe atom subjected to a mid-infrared laser field. It is found that the energy of LES becomes lower for shorter pulse duration. Further analysis shows that in the case of multi-cycle laser field, the LESn structure is closely related to the number of times of forward scattering and the initial transverse momentum. In the case of few-cycle laser pulse, the carrier-envelope phase (CEP) dependence of the peak position of LES is mainly due to the CEP dependence of the influence of both vector-potential of the laser field and the Coulomb potential. In addition, the bunching effect of electrons, caused by Coulomb potential, is the main reason for the formation of LES.

Strong-field atomic physics meets 229Th nuclear physics

We show how two apparently unrelated research areas, namely, strong-field atomic physics and 229Th nuclear physics, are connected. The connection is possible due to the existence of a very low-lying excited state of the 229Th nucleus, which is only about 8 eV above the nuclear ground state. The connection is physically achieved through an electron recollision process, which is the core process of strong-field atomic physics. The laser-driven recolliding electron is able to excite the nucleus, and a simple model is presented to explain this recollision-induced nuclear excitation process. The connection of these two research areas provides novel opportunities for each area and intriguing possibilities from the direct three-partite interplay between atomic physics, nuclear physics, and laser physics.

Exciting the Isomeric ^{229}Th Nuclear State via Laser-Driven Electron Recollision.

We propose a new approach to excite the isomeric ^{229}Th nuclear state, which has attracted much attention recently as a potential "nuclear clock." Our approach is based on a laser-driven electron recollision process, the core process of strong-field atomic physics. Bringing together knowledge of recollision physics and of the related nuclear physics, we calculate the isomeric excitation probability. This new approach does not require precise knowledge of the energy of the isomeric state. The excitation is well timed which may be exploited to control the coherence of the isomeric state. Experimental realization is within reach using tabletop laser systems.

Long-Range Coulomb Effect in Intense Laser-Driven Photoelectron Dynamics

In strong field atomic physics community, long-range Coulomb interaction has for a long time been overlooked and its significant role in intense laser-driven photoelectron dynamics eluded experimental observations. Here we report an experimental investigation of the effect of long-range Coulomb potential on the dynamics of near-zero-momentum photoelectrons produced in photo-ionization process of noble gas atoms in intense midinfrared laser pulses. By exploring the dependence of photoelectron distributions near zero momentum on laser intensity and wavelength, we unambiguously demonstrate that the long-range tail of the Coulomb potential (i.e., up to several hundreds atomic units) plays an important role in determining the photoelectron dynamics after the pulse ends.

Soliton dynamics in the multiphoton plasma regime

Solitary waves have consistently captured the imagination of scientists, ranging from fundamental breakthroughs in spectroscopy and metrology enabled by supercontinuum light, to gap solitons for dispersionless slow-light, and discrete spatial solitons in lattices, amongst others. Recent progress in strong-field atomic physics include impressive demonstrations of attosecond pulses and high-harmonic generation via photoionization of free-electrons in gases at extreme intensities of 1014 W/cm2. Here we report the first phase-resolved observations of femtosecond optical solitons in a semiconductor microchip, with multiphoton ionization at picojoule energies and 1010 W/cm2 intensities. The dramatic nonlinearity leads to picojoule observations of free-electron-induced blue-shift at 1016 cm−3 carrier densities and self-chirped femtosecond soliton acceleration. Furthermore, we evidence the time-gated dynamics of soliton splitting on-chip, and the suppression of soliton recurrence due to fast free-electron dynamics. These observations in the highly dispersive slow-light media reveal a rich set of physics governing ultralow-power nonlinear photon-plasma dynamics.

Vacuum-ultraviolet frequency combs from below-threshold harmonics

Recent demonstrations of high-harmonic generation (HHG) at very high repetition frequencies (~100 MHz) may allow for the revolutionary transfer of frequency combs to the vacuum ultraviolet (VUV). This advance necessitates unifying optical frequency comb technology with strong-field atomic physics. While strong-field studies of HHG have often focused on above-threshold harmonic generation (photon energy above the ionization potential), for VUV frequency combs an understanding of below-threshold harmonic orders and their generation process is crucial. Here we present a new and quantitative study of the harmonics 7-13 generated below and near the ionization threshold in xenon gas. We show multiple generation pathways for these harmonics that are manifested as on-axis interference in the harmonic yield. This discovery provides a new understanding of the strong-field, below-threshold dynamics under the influence of an atomic potential and allows us to quantitatively assess the achievable coherence of a VUV frequency comb generated through below threshold harmonics. We find that under reasonable experimental conditions temporal coherence is maintained. As evidence we present the first explicit VUV frequency comb structure beyond the 3rd harmonic.

Atoms in high intensity mid-infrared pulses

The recent development of intense, ultrashort, table top lasers in the mid-infrared opens new avenues for research in strong field atomic physics. Electrons submitted to such radiation acquire huge quiver energies, even at moderate intensity and interesting properties arise: first, the wavelength offers a convenient experimental knob to tune the ionisation regime by controlling the Keldysh parameter. Second, many processes like above-threshold ionisation or high harmonic generation, whose characteristics depend directly on this energy, can be pushed to unprecedented limits. Third, the wavelength controls the spectral phase of the harmonics and hence the possibility to improve the generation of pulses in the attosecond regime. Recent studies of rare gas and alkali atoms' photoelectron spectra and harmonic generation at 2 and 3.6 μm have begun to confirm the theoretical predictions. However, unexpected features have also been found showing that strong field interaction still keeps some secrets after more than 40 years of investigation.

Quantum-mechanical analogy of beam propagation in waveguides with a bent axis: dynamic-mode stabilization and radiation-loss suppression.

Wave propagation in an optical waveguide with a bent axis is studied under the scalar and paraxial wave approximations, and the quantum mechanical analogy with the electron dynamics in an atomic potential interacting with an intense electromagnetic field is highlighted. In particular we show that for a truncated parabolic waveguide with a periodically curved axis, a dynamic mode splitting with reduced radiation losses can be observed, which is fully analogous to the phenomenon of wave packet dichotomy and ionization quenching found in strong-field atomic physics.

Proposal for New Storage Ring Activities at GSI

The paper gives an overview on a proposed major facility at GSI and its main scientific aims. By means of innovative accelerators and storage-cooler rings, high-energy beams of all elements as well as rare isotope- and antiproton beams of unprecedented intensity and quality can be provided. There with long-standing questions in different fields of physics can be addressed, most of them related to the basic structure of hadronic matter, such as: nuclear structure far from stability; explosive stellar nucleosynthesis; fundamental symmetries, in particular of weak interaction; origin of hadronic mass; restoration of chiral symmetry; quark deconfinement at high density; plasmas under extreme conditions; strong-field atomic physics. A unique feature of the new facility is that different research programs can be performed to a large extent in truly parallel operation.