26 research outputs found
Polarization tagging of two-photon double ionization by elliptically polarized XUV pulses
We explore the influence of elliptical polarization on the (non)sequential two-photon double ionization of atomic helium with ultrashort extreme ultraviolet (XUV) light fields using time-dependent full ab initio simulations. The energy and angular distributions of photoelectrons are found to be strongly dependent on the ellipticity. The correlation minimum in the joint angular distribution becomes more prominently visible with increasing ellipticity. In a pump-probe sequence of two subsequent XUV pulses with varying ellipticities, polarization tagging allows us to discriminate between sequential and nonsequential photoionization. This clear separation demonstrates the potential of elliptically polarized XUV fields for improved control of electronic emission processes.This work was supported by the WWTF through Project No. MA14-002, and the FWF through Projects No. FWF-SFB041-VICOM, No. FWF-SFB049-NEXTlite, and No. FWF-W1243-Solids4Fun, as well as the IMPRS-APS. J.F. acknowledges support by the Spanish MINECO through a RamĂłn y Cajal grant and the “MarĂa de Maeztu” program for Units of Excellence in R&D (MDM-2014-0377
Quasiphoton at the Subcycle Level in Strong-Field Ionization
Photon is an energy quanta of light that does not exist at the
sub-optical-cycle level. Exploiting the dynamical rotational symmetry of
circularly or elliptically polarized light pulses, however, we demonstrate the
existence of quasiphotons down to the subcycle level. We illustrate the concept
of quasiphotons in strong-field ionization through the correlated spectrum of
angular momentum and energy (SAME) of photoelectrons, both at the tunnel exit
and in the asymptotic region. Moreover, we propose a protocol based on electron
vortices to directly visualize the existence of quasiphotons. Our work paves
the pathway towards a deeper understanding of fundamental light-matter
interactions with photonic characteristics on the subcycle scale.Comment: 6 pages, 4 figure
Ionization Induced by the Ponderomotive Force in Intense and High-Frequency Laser Fields
Atomic stabilization is a universal phenomenon that occurs when atoms
interact with intense and high-frequency laser fields. In this work, we
systematically study the influence of the ponderomotive (PM) force, present
around the laser focus, on atomic stabilization. We show that the PM force
could induce tunneling and even over-barrier ionization to the otherwise
stabilized atoms. Such effect may overweight the typical multiphoton ionization
under moderate laser intensities. Our work highlights the importance of an
improved treatment of atomic stabilization that includes the influence of the
PM force
Theory of Subcycle Linear Momentum Transfer in Strong-Field Tunneling Ionization
Interaction of a strong laser pulse with matter transfers not only energy but also linear momentum of the photons. Recent experimental advances have made it possible to detect the small amount of linear momentum delivered to the photoelectrons in strong-field ionization of atoms. We present numerical simulations as well as an analytical description of the subcycle phase (or time) resolved momentum transfer to an atom accessible by an attoclock protocol. We show that the light-field-induced momentum transfer is remarkably sensitive to properties of the ultrashort laser pulse such as its carrier-envelope phase and ellipticity. Moreover, we show that the subcycle-resolved linear momentum transfer can provide novel insights into the interplay between nonadiabatic and nondipole effects in strong-field ionization. This work paves the way towards the investigation of the so-far unexplored time-resolved nondipole nonadiabatic tunneling dynamics. © 2020 authors
Wave-Packet Surface Propagation for Light-Induced Molecular Dissociation
Recent advances in laser technology have enabled tremendous progress in
photochemistry, at the heart of which is the breaking and formation of chemical
bonds. Such progress has been greatly facilitated by the development of
accurate quantum-mechanical simulation method, which, however, does not
necessarily accompany clear dynamical scenarios and is rather often a black
box, other than being computationally heavy. Here, we develop a wave-packet
surface propagation (WASP) approach to describe the molecular bond-breaking
dynamics from a hybrid quantum-classical perspective. Via the introduction of
quantum elements including state transitions and phase accumulations to the
Newtonian propagation of the nuclear wave-packet, the WASP approach naturally
comes with intuitive physical scenarios and accuracies. It is carefully
benchmarked with the H2+ molecule and is shown to be capable of precisely
reproducing experimental observations. The WASP method is promising for the
intuitive visualization of strong-field molecular dynamics and is
straightforwardly extensible toward complex molecules.Comment: 24 pages, 4 figure
A Novel Nonlinear Optical Limiter Based on Stimulated Brillouin Scattering in Highly-Nonlinear Fiber
A novel nonlinear optical limiter (NOL) based on stimulated Brillouin scattering (SBS) in highly nonlinear fiber was proposed and experimentally demonstrated at 1550 nm wavelength. The nonlinear optical limiting effects of HNLF were characterized and demonstrated theoretically and experimentally. In a proof-of-concept experiment, we verified that the NOL based on a 50 m HNLF has excellent limiting performance due to its small effective area and high Brillouin gain coefficient. The linear transmittance and lowest nonlinear transmittance of the NOL were 87.5% and 11.9%, respectively
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Theoretical Studies of Ultrafast Correlated Electron Dynamics in Single and Double Photoionization
Recent advances in laser technology have led to the generation of attosecond laser pulses, whose duration is in the range of the natural time scale of electron dynamics, and thus make the observation and even control of electron dynamics in atoms and molecules possible. While the single-electron dynamics is more thoroughly studied, the correlated dynamics of two electrons is less understood, especially in the context of resolving the ultrafast temporal information in double photoionization. In this thesis, we first study the energy exchange via electron correlation upon photon absorption over large distances in the two-site double photoionization of the helium dimer, which is found to be a two-step process. In the first step, one electron in one atom absorbs the photon and gets ionized. In the second step, this electron propagates towards the neighboring atom and knocks out the other electron. We then introduce the Hamiltonian reduction method to further study the effects of different interactions in the single and double photoionization of the helium dimer. Next, we analyze the selection rules for the emission of two electrons from the helium atom, the helium dimer, and general molecules following the absorption of a few photons in an intense laser field. In particular, the back-to-back emission of the two electrons with equal energy sharing is either suppressed or not depending on the number of photons absorbed from the field.
Finally, we study the time delay between the single and double photoionization processes. We first propose a self-consistent-time method to account for the Coulomb-laser coupling effect and obtain the intrinsic photoabsorption time delay measured by the attosecond streak camera. We then proceed to time resolve the correlated emission of two electrons in the knockout process of the helium dimer with respect to the first step of single ionization