Strong Circularly Polarized Laser Field Research Articles

Ionization and excitation of low-lying circular states of the hydrogen atom in strong circularly polarized laser fields

We perform ab initio calculations for the hydrogen atom initially in one of the six circular bound states with the principal quantum numbers n = 2, 3 and 4, irradiated by a short circularly polarized laser pulse of 400 nm. The field propagates in the direction parallel to the z-component of the angular momentum of the atom. We investigate probabilities for the atom to ionize or to get on some bound (excited) state or to remain in the initial state after the end of the laser pulse. In most cases, we find pronounced differences in ionization probabilities for atoms in states having different signs of magnetic quantum number. Usually electrons corotating (with respect to the laser field) ionize faster than their counter-rotating equivalents. We have found important difference in the behavior of the excitation (as a function of the peak laser intensity) for initial states with n = 2 and other n (3 or 4). For higher n the excitation is always weaker than the ionization and starts for higher intensities. For n = 2 the strong excitation appears before strong ionization due to large probability of one-photon absorption combined with population of many bound states having principal quantum numbers n from 3 up to several dozen. Quite accurate analysis of the excitation process is presented.

Absorption and opacity threshold for a thin foil in a strong circularly polarized laser field.

We show that a commonly accepted transparency threshold for a thin foil in a strong circularly polarized normally incident laser pulse needs a refinement. We present an analytical model that correctly accounts for laser absorption. The refined threshold is determined not solely by the laser amplitude, but by other parameters that are equally or even more important. Our predictions are in perfect agreement with particle-in-cell simulations. The refined criterion is crucial for configuring laser plasma experiments in the high-field domain. In addition, an opaque foil steepens the pulse front, which can be important for numerous applications.

Revealing the multiorbital and excitation effects in dissociative double ionization of CO molecules with angular streaking measurement

SynopsisWe investigate the sequential double ionization dynamics for the dissociative channel C+ + O+ of CO molecules in strong circularly polarized laser fields. By using four-particle coincident measurement, the obtained ionic angular emissions and molecular frame photoelectron angular distributions of each ionization steps for various pathways reveals the significant effects of multiorbital ionization and laser-driven excitation.

Multiorbital and excitation effects on dissociative double ionization of CO molecules in strong circularly polarized laser fields

Multiorbital and excitation effects on dissociative double ionization of CO molecules in strong circularly polarized laser fields

Attosecond control of spin polarization in electron–ion recollision driven by intense tailored fields

Tunnel ionization of noble gas atoms driven by a strong circularly polarized laser field in combination with a counter-rotating second harmonic generates spin-polarized electrons correlated to the spin-polarized ionic core. Crucially, such two-color field can bring the spin-polarized electrons back to the parent ion, enabling the scattering of the spin-polarized electron on the spin-polarized parent ion. Here we show how one can control the degree of spin polarization as a function of electron energy and recollision time by tuning the laser parameters, such as the relative intensities of the counter-rotating fields. The attosecond precision of the control over the degree of spin polarization opens the door for attosecond control and spectroscopy of spin-resolved dynamics.

Producing spin-polarized photoelectrons by using the momentum gate in strong-field ionization experiments

The ionization of the prealigned nitric oxide molecule by strong circularly polarized laser fields is studied via theoretical simulations of the spin-resolved photoelectron momentum distributions by solving numerically the three-dimensional time-dependent Schr\odinger equation. Due to the spin-orbit entanglement in the ground state of nitric oxide and the sensitivity of the tunnel ionization of its doubly degenerate valence $2{\ensuremath{\pi}}_{\ifmmode\pm\else\textpm\fi{}}$ orbitals carrying opposite electron ring currents to the sense of the laser-field rotation, the momentum-resolved spin-polarized photoelectrons are produced. We show that the spin polarization exhibits strong dependence on the kinetic energy as well as the emitting angle of the photoelectron. In addition to the laser control, the momentum gate in strong-field experiments would enable full control of the spin polarization.

Spin-polarized photoelectrons produced by strong-field ionization of randomly aligned nitric oxide

We investigate the effect of the molecular alignment of nitric oxide (NO) on nonadiabatic tunnel ionization of degenerate valence orbitals in strong circularly polarized laser fields and on spin polarization of photoelectrons. Our numerical study shows that not only for the alignment parallel to the laser propagation axis [Liu, K.; Barth, I. Phys. Rev. A 2016, 94, 043402] but also for arbitrary alignment angles except for perpendicular alignment, the counter-rotating molecular orbital with respect to the circular polarization of the laser field projected on the plane perpendicular to the molecular axis is ionized more easily. Due to the nonadiabatic effect and the nodal structure of the valence orbitals of NO, the ionization maxima for the and orbitals in right circularly polarized laser fields are obtained for the molecular orientations at the polar angles of around and , respectively. Considering the spin-orbital entanglement in the doubly degenerate electronic ground state of NO, the spin-polarization of photoelectrons is high for parallel alignment, decreases upon increasing the polar angle and vanishes for perpendicular alignment. Averaging over all alignment angles, non-zero spin-polarization of photoelectrons for randomly aligned NO is preserved.

Nonadiabatic tunnel ionization of current-carrying orbitals of prealigned linear molecules in strong circularly polarized laser fields

Nonadiabatic tunnel ionization of current-carrying orbitals of prealigned linear molecules in strong circularly polarized laser fields

Ionization and excitation of the excited hydrogen atom in strong circularly polarized laser fields

In the recent work of Herath et al. [T. Herath, L. Yan, S. K. Lee, and W. Li, Phys. Rev. Lett. 109, 043004 (2012)] the first experimental observation of a dependence of strong-field ionization rate on the sign of the magnetic quantum number $m$ [of the initial bound state $(n,l,m)]$ was reported. The experiment with nearly circularly polarized light could not distinguish which sign of $m$ favors faster ionization. We perform ab initio calculations for the hydrogen atom initially in one of the four bound substates with the principal quantum number $n=2$, and irradiated by a short circularly polarized laser pulse of $800\phantom{\rule{0.28em}{0ex}}\mathrm{nm}$. In the intensity range of ${10}^{12}\ensuremath{-}{10}^{13}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ excited bound states play a very important role, but also up to some ${10}^{15}\phantom{\rule{0.28em}{0ex}}\mathrm{W}/\mathrm{c}{\mathrm{m}}^{2}$ they cannot be neglected in a full description of the laser-atom interaction. We explore the region that with increasing intensity switches from multiphoton to over-the-barrier ionization and we find, unlike in tunneling-type theories, that the ratio of ionization rates for electrons initially counter-rotating and corotating (with respect to the laser field) may be higher or lower than 1.

Numerical verification of the theory of nonadiabatic tunnel ionization in strong circularly polarized laser fields

We verify the theory of nonadiabatic ionization of degenerate valence orbitals in strong circularly polarized laser fields by numerically solving the two-dimensional time-dependent Schrödinger equation for an effective one-electron potential of neon. The numerically calculated ionization ratios of the and orbitals agree well with the theoretical results (i.e., the counter-rotating electron tunnels more easily). However, for strong laser pulses and low laser frequencies, the adiabatic laser-dressed orbitals play an important role. In a Floquet treatment of a three-level model, we find that in this regime the ionization ratio of initial and orbitals depends strongly on the orbital energy order of valence s and orbitals. We also show that the emission angles of valence and electrons are different and should be observable in attoclock experiments.

Hole dynamics and spin currents after ionization in strong circularly polarized laser fields

We apply the time-dependent analytical R-matrix theory to develop a movie of hole motion in a Kr atom upon ionization by strong circularly polarized field. We find rich hole dynamics, ranging from rotation to swinging motion. The motion of the hole depends on the final energy and the spin of the photoelectron and can be controlled by the laser frequency and intensity. Crucially, hole rotation is a purely non-adiabatic effect, completely missing in the framework of quasistatic (adiabatic) tunneling theories. We explore the possibility to use hole rotation as a clock for measuring ionization time. Analyzing the relationship between the relative phases in different ionization channels we show that in the case of short-range electron-core interaction the hole is always initially aligned along the instantaneous direction of the laser field, signifying zero delays in ionization. Finally, we show that strong-field ionization in circular fields creates spin currents (i.e. different flow of spin-up and spin-down density in space) in the ions. This phenomenon is intimately related to the production of spin-polarized electrons in strong laser fields Barth and Smirnova (2013 Phys. Rev. A 88 013401). We demonstrate that rich spin dynamics of electrons and holes produced during strong field ionization can occur in typical experimental conditions and does not require relativistic intensities or strong magnetic fields.

How Key Periodic Orbits Drive Recollisions in a Circularly Polarized Laser Field

We show that a family of key periodic orbits drives the recollision process in a strong circularly polarized laser field. These orbits, coined recolliding periodic orbits, exist for a wide range of parameters, and their relative influence changes as the laser and atomic parameters are varied. We find the necessary conditions for recollision-driven nonsequential double ionization to occur. The outlined mechanism is universal in that it applies equally well beyond atoms: The internal structure of the target species plays a minor role in the recollision process.

Comparison of two forms of the S-matrix element of strong-field photoionization: II. (n, l, m) = (2, 1, ±1) states

In our previously published work (Bauer 2008 J. Phys. B: At. Mol. Opt. Phys. 41 185003) we numerically compared ionization rates and photoelectron energy (or momentum) spectra resulting from the so-called length gauge (LG) and velocity gauge (VG) forms of the Keldysh–Faisal–Reiss theory for a strong circularly polarized laser field. Our investigations concerned initial (bound) states of a hydrogen atom with the principal quantum number equal to 1 or 2. However, some of these states were not taken into account in this work, but only their linear combinations (quantum-mechanical superpositions). The main purpose of the present work is to fill this gap. It is appropriate to add that excited bound states (of the hydrogen atom) considered here are easier to achieve from an experimental point of view. As in our previous work, we find substantial differences in the shape of photoelectron energy spectra for both (LG and VG) theories. Differently from the VG theory, the LG one shows a multipeak envelope in photoelectron energy spectra for most initial states of the hydrogen atom. We give examples showing that the multipeak effect may be observed also in the polarization plane of the laser field. The field parameters (frequency and intensity) applied by us in the calculations indicate that one should be able to observe experimentally the above-mentioned effect in the near future.

Multipeak envelope of the above-threshold-ionization energy spectrum in a strong circularly polarized laser field

We develop a well-known strong-field approximation in the length gauge for a two-dimensional model atom in a strong circularly polarized laser field. For excited initial states (with the principal quantum number $n=2$) outgoing photoelectrons usually show energy spectra with an envelope having two or three peaks: one always below and one always above the ponderomotive energy ${U}_{P}$. In contrast, for the ground state (with $n=1$) there is always a single peak near ${U}_{P}$. Our numerical calculations show that the multipeak effect appears for sufficiently high laser intensities $I$ and is more pronounced for higher laser frequencies $\ensuremath{\omega}$. We believe that the field parameters $\ensuremath{\omega}$ and $I$, for which the multipeak envelope in the energy spectrum of above-threshold ionization exists, will enable its experimental observation in the near future.

Photodetachment in a strong circularly polarized laser field

The saddle-point analysis of the transition amplitude shows that the quantum interference effect does not occur in the direct process of photodetachment by a circularly polarized laser field. This fact is interpreted in terms of classical electron trajectories. Comparison between the length and velocity gauges is performed by simulating spectra of photoelectrons produced in a focus of a laser pulse. A substantial discrepancy is found in the kinetic energy distribution of photoelectrons. At low energies, only the predictions in the length gauge are consistent with the Wigner threshold law.

Reduction of the Thompson scattering cross section in a strong circularly polarized light field in a plasma with the change of its spectrum. “quantum-classical” electron

It is shown that in a strong circularly polarized laser field a classical electron motion around ions can occur. The scattering of these electrons in a plasma has the Thompson cross section in the limit of strongs field only and for a subrelativistic motion of the electrons. There are non-ion satellites apart from the basic frequency in the scattering spectrum.