Real-Time Observation of Instantaneous ac Stark Shift of a Vacuum Using a Zeptosecond Laser Pulse

Based on the numerical solution of the time-dependent Dirac equation,we propose a method to observe in real time the ac Stark shift of a vacuum driven by an ultra-intense laser field.By overlapping the ultra-intense pump pulse with another zeptosecond probe pulse whose photon energy is smaller than 2mc^(2),electron–positron pair creation can be controlled by tuning the time delay between the pump and probe pulses.Since the pair creation rate depends sensitively on the instantaneous vacuum potential,one can reconstruct the ac Stark shift of the vacuum potential according to the time-delay-dependent pair creation rate.

Photonuclear production of medical isotopes^(62,64)Cu using intense laser-plasma electron source

^(62,64)Cu are radioisotopes of medical interest that can be used for positron emission tomography(PET)imaging.Moreover,64Cu hasβ−decay characteristics that allowfor targeted radiotherapy of cancer.In the present work,a novel approach to experimentally demonstrate the production of ^(62,64)Cu isotopes fromphotonuclear reactions is proposed in which large-current laser-based electron(e−)beams are generated fromthe interaction between sub-petawatt laser pulses and near-critical-density plasmas.According to simulations,at a laser intensity of 3.431021 W/cm2,a dense e−beamwith a total charge of 100 nCcan be produced,and this in turn produces bremsstrahlung radiation of the order of 1010 photons per laser shot,in the region of the giant dipole resonance.The bremsstrahlung radiation is guided to a natural Cu target,triggering photonuclear reactions to produce themedical isotopes ^(62,64)Cu.An optimal target geometry is employed to maximize the photoneutron yield,and ^(62,64)Cuwith appropriate activities of 0.18 GBq and 0.06 GBq are obtained for irradiation times equal to their respective half-livesmultiplied by three.The detection of the characteristic energy for the nuclear transitions of ^(62,64)Cu is also studied.The results of our calculations support the prospect of producing PET isotopes with gigabecquerel-level activity(equivalent to the required patient dose)using upcoming high-intensity laser facilities.

Tunneling exits of H_2^+ in strong laser fields

Different from atoms, the multicenter of the Coulombic potentials in molecules makes the tunneling ionization complex,and the electron tunnels out the laser-dressed Coulomb potential with a complex structure. We study tunneling exits of H2^＋ at large internuclear distance in strong laser fields by numerically simulating the time-dependent Schrodinger equation plus a classical backward propagation of the ionized wave packet. This study strengthens the understanding of molecular tunneling ionization in strong laser fields.

EMP control and characterization on high-power laser systems

Giant electromagnetic pulses(EMP) generated during the interaction of high-power lasers with solid targets can seriously degrade electrical measurements and equipment. EMP emission is caused by the acceleration of hot electrons inside the target, which produce radiation across a wide band from DC to terahertz frequencies. Improved understanding and control of EMP is vital as we enter a new era of high repetition rate, high intensity lasers(e.g. the Extreme Light Infrastructure).We present recent data from the VULCAN laser facility that demonstrates how EMP can be readily and effectively reduced. Characterization of the EMP was achieved using B-dot and D-dot probes that took measurements for a range of different target and laser parameters. We demonstrate that target stalk geometry, material composition, geodesic path length and foil surface area can all play a significant role in the reduction of EMP. A combination of electromagnetic wave and 3 D particle-in-cell simulations is used to inform our conclusions about the effects of stalk geometry on EMP,providing an opportunity for comparison with existing charge separation models.

Magnetic field annihilation and reconnection driven by femtosecond lasers in inhomogeneous plasma

The process of fast magnetic reconnection driven by intense ultra-short laser pulses in underdense plasma is investigated by particle-in-cell simulations. In the wakefield of such laser pulses, quasi-static magnetic fields at a few mega-Gauss are generated due to nonvanishing cross product ▽(n/) × p. Excited in an inhomogeneous plasma of decreasing density, the quasi-static magnetic field structure is shown to drift quickly both in lateral and longitudinal directions. When two parallel-propagating laser pulses with close focal spot separation are used, such field drifts can develop into magnetic reconnection(annihilation) in their overlapping region, resulting in the conversion of magnetic energy to kinetic energy of particles. The reconnection rate is found to be much higher than the value obtained in the Hall magnetic reconnection model. Our work proposes a potential way to study magnetic reconnection-related physics with short-pulse lasers of terawatt peak power only.

Low-Energy Protons in Strong-Field Dissociation of H_(2)^(+) via Dipole-Transitions at Large Bond Lengths

More than ten years ago,the observation of the low-energy structure in the photoelectron energy spectrum,regarded as an“ionization surprise,”has overthrown our understanding of strong-field physics.However,the similar low-energy nuclear fragment generation from dissociating molecules upon the photon energy absorption,one of the well-observed phenomena in light-molecule interaction,still lacks an unambiguous mechanism and remains mysterious.Here,we introduce a time-energy-resolved manner using a multicycle near-infrared femtosecond laser pulse to identify the physical origin of the light-induced ultrafast dynamics of molecules.By simultaneously measuring the bond-stretching times and photon numbers involved in the dissociation of H_(2)^(+) driven by a polarization-skewed laser pulse,we reveal that the low-energy protons(below 0.7 eV)are produced via dipole-transitions at large bond lengths.The observed low-energy protons originate from strong-field dissociation of high vibrational states rather than the low ones of H_(2)^(+) cation,which is distinct from the well-accepted bond-softening picture.Further numerical simulation of the time-dependent Schrödinger equation unveils that the electronic states are periodically distorted by the strong laser field,and the energy gap between the field-dressed transient electronic states may favor the one-or three-photon transitions at the internuclear distance larger than 5 a.u.The time-dependent scenario and our time-energy-resolved approach presented here can be extended to other molecules to understand the complex ultrafast dynamics.

Attosecond Delays of High-Harmonic Emissions from Hydrogen Isotopes Measured by XUV Interferometer

High-harmonic spectroscopy can access structural and dynamical information on molecular systems encoded in the amplitude and phase of high-harmonic generation(HHG)signals.However,measurement of the harmonic phase is a daunting task.Here,we present a precise measurement of HHG phase difference between two isotopes of molecular hydrogen using the advanced extreme-ultraviolet(XUV)Gouy phase interferometer.The measured phase difference is about 200 mrad,corresponding to~3 attoseconds(1 as=10−18 s)time delay which is nearly independent of harmonic order.The measurements agree very well with numerical calculations of a four-dimensional time-dependent Schödinger equation.Numerical simulations also reveal the effects of molecular orientation and intramolecular two-center interference on the measured phase difference.This technique opens a new avenue for measuring the phase of harmonic emission for different atoms and molecules.Together with isomeric or isotopic comparisons,it also enables the observation of subtle effects of molecular structures and nuclear motion on electron dynamics in strong laser fields.

Rabi oscillations in a stretching molecule

Rabi oscillation is an elementary laser-driven physical process in atoms and artificial atoms from solid-state systems,while it is rarely demonstrated in molecules.Here,we investigate the bond-length-dependent Rabi oscillations with varying Rabi frequencies in strong-laser-field dissociation of H2+.The coupling of the bond stretching and Rabi oscillations makes the nuclei gain different kinetic energies while the electron is alternatively absorbing and emitting photons.The resulting proton kinetic energy spectra show rich structures beyond the prediction of the Floquet theorem and the well-accepted resonant one-photon dissociation pathway.Our study shows that the laser-driven Rabi oscillations accompanied by nuclear motions are essential to understanding the bond-breaking mechanism and provide a time-resolved perspective to manipulate rich dynamics of the strong-laser-field dissociation of molecules.

Ultrafast Two-Electron Orbital Swap in Li Initiated by Attosecond Pulses

A universal mechanism of ultrafast 2-electron orbital swap is discovered through 2-photon sequential double ionization of Li.After a 1s electron in Li is ionized by absorbing an extreme ultraviolet photon,the other 2 bound electrons located on 2 different shells have either parallel or antiparallel spin orientations.In the latter case,these 2 electrons are in the superposition of the singlet and triplet states with different energies,forming a quantum beat and giving rise to the 2-electron orbital swap with a period of several hundred attoseconds.The orbital swap mechanism can be used to manipulate the spin polarization of photoelectron pairs by conceiving the attosecond-pump attosecond-probe strategy and thus serves as a knob to control spin-resolved multielectron ultrafast dynamics.