Circularly Polarized Laser Field Research Articles

Terahertz dependence on the laser phase and wavelength in two-color circularly polarized laser fields

Two-color circularly polarized laser fields with the same helicity (CP-S) are an appealing type of driving field that is essential for generating strong terahertz (THz) radiation. In this work, we theoretically investigate how the phase delay between two components in a CP-S field and their wavelength affect THz radiation yield with a photocurrent model. Simulations show that, as the laser wavelength increases, the THz yield becomes more and more phase dependent, which is maximized when the relative phase is 0 and minimized when the relative phase is π. Such a phase dependence is completely different from the commonly used two-color linearly polarized laser pulse with parallel polarizations (LP-P). In addition, in the CP-S field, the THz radiation energy may exhibit a wavelength scaling λ α (λ is the fundamental wavelength) with a bigger exponent than in the LP-P field at longer wavelengths. Our findings have important implications for the generation of powerful THz radiation via femtosecond laser interaction with gasses.

Orientation Dependence of High-Order Harmonic Generation from HeH2+ in a Corotating Two-Color Circularly Polarized Laser Field

By numerically solving the time-dependent Schrödinger equation, we study high-order harmonic generation from the asymmetric diatomic molecule HeH2+ in a corotating two-color circularly polarized laser field. Our results reveal a strong correlation between the molecule orientation and the harmonic yield. The harmonics in the plateau region can achieve an intensity modulation of one to two orders of magnitude with the change in the orientation angle. Through the time-dependent evolution of ionized electron wave packets combined with the analysis of the transition dipole moment between the continuum and bound states, the modulation of the harmonic strength may be attributed to the difference in the recollision angle of ionized electron wave packets relative to the molecules. In addition, the harmonic ellipticity is also affected by the molecular orientation. Notably, we found that the harmonic with greater ellipticity and higher intensity can be obtained with an orientation angle of 147°. These findings open up new avenues for achieving enhanced efficiency, the near-circular polarization of harmonics, and precise control over harmonic polarization states.

Nonsequential double ionization of Ar atoms in counter-rotating two-color elliptically polarized laser fields

Electron correlation behaviors and recollision dynamics in nonsequential double ionization (NSDI) of Ar atoms in counter-rotating two-color elliptically polarized (TCEP) laser fields are investigated by using the classical ensemble model. The combined electric field in counter-rotating TCEP laser pulses traces out a trefoil pattern, i.e. the waveform in a period shows three leaves in different directions, and each leaf is called a lobe. Unlike counter-rotating two-color circularly polarized laser field, the combined electric field has no spatial symmetry. The amplitudes of the three field lobes and the angles between them are different. Thus, the returning electron mainly returns to the parent ion from one direction, and the electron momentum distributions show strong asymmetry. Numerical results show that the NSDI yield gradually decreases as the ellipticity increases, and the correlated behavior of the correlated electron momentum along the long axis of the laser polarization plane gradually evolves from correlation behavior mainly located in the first quadrant and the third quadrant to anti-correlation behavior mainly located in the second quadrant and fourth quadrant. In order to further understand the correlation behaviors of electron pairs, different characteristic times in the NSDI processes are discussed, respectively. It is found that single ionization events and recollision events gradually decrease, but single ionization time and recollision time change slightly. This may be the main reason for the decrease in NSDI yield. And as the ellipticity increases, the traveling time and the recollision energy gradually decrease, while the delay time increases. Therefore, we can conclude that ellipticity may be responsible for the NSDI process. In addition, further analysis finds that the shape of the trajectory becomes more and more triangular as the ellipticity increases due to the counter-rotating TCEP laser fields of the specific dynamical symmetries of the total net electric field. And it is found that whether it is a “short trajectory” or a “long trajectory”, more populations move to the second quadrant and the fourth quadrant as the ellipticity increases. The results show that increasing the ellipticity will gradually change the two electrons from emitting in the same direction to emitting in the opposite direction. This well demonstrates that both ellipticity and travelling time are responsible for the formation of the electron momentum distribution at the recollision time, meaning that both of them affect the emitted directions of both electrons.

Modulation of High-Order Harmonic Generation from a Monolayer ZnO by Co-rotating Two-Color Circularly Polarized Laser Fields

By numerically solving the two-dimensional semiconductor Bloch equation, we study the high-order harmonic emission of a monolayer ZnO under the driving of co-rotating two-color circularly polarized laser pulses. By changing the relative phase between the fundamental frequency field and the second one, it is found that the harmonic intensity in the platform region can be significantly modulated. In the higher order, the harmonic intensity can be increased by about one order of magnitude. Through time-frequency analysis, it is demonstrated that the emission trajectory of monolayer ZnO can be controlled by the relative phase, and the harmonic enhancement is caused by the second quantum trajectory with the higher emission probability. In addition, near-circularly polarized harmonics can be generated in the co-rotating two-color circularly polarized fields. With the change of the relative phase, the harmonics in the platform region can be altered from left-handed near-circularly polarization to right-handed one. Our results can obtain high-intensity harmonic radiation with an adjustable ellipticity, which provides an opportunity for syntheses of circularly polarized attosecond pulses.

Nonsequential double ionization channels control of CO2 molecules with counter-rotating two-color circularly polarized laser field by laser wavelength

Abstract Using the classical ensemble model, we investigate the effect of laser wavelength on the electron dynamics process of nonsequential double ionization (NSDI) for linear triatomic molecules driven by a counter-rotating two-color circularly polarized laser field. Based on the delay time between recollision and final double ionization, two particular ionization channels are separated: recollision-impact ionization (RII) and recollision-induced excitation with subsequent ionization (RESI). Numerical results show that with the increase of the laser wavelength, the triangle structure of the ion momentum distribution becomes more obvious, which indicates that the electron–electron correlation of NSDI is enhanced. In addition, we find that the ratio of the RESI channel gradually decreases with the increase of the laser wavelength, while the ratio of the RII channel is opposite. However, the dominant channel is still RESI. It means that the two ionization channels can be controlled effectively by changing the laser wavelength.

Shepherd electron effects in multiple ionization of rubidium by circularly polarized intense laser fields

Nonsequential double/multiple ionization (NSDI/NSMI) of atoms in strong laser fields is a paradigm system for studying field-intervened electron-electron correlation. However, there exists a long-standing debated topic as to how NSDI/NSMI is triggered in a circularly-polarized laser field. In this contribution, it is shown counter-intuitively that the NSMI of alkali atoms is strongly enhanced due to a mechanism induced by their intrinsic shell structure, coined as the shepherd electron effect. Specifically, we find that the early-released outermost electron (the shepherd electron) can be stabilized on the Rydberg states even after dislodging some other inner-shell electrons and thus lead to the emergence of a transient hollow atom. Experimental signatures of the shepherd electron effect and its real-time observation with the attoclock technique are discussed. These results have substantially enriched our understanding of NSMI, beyond the well-accepted cascade recollision picture. They might have applications in coherent extreme-ultraviolet (XUV) light amplification.

The Influence of the Effective Ionization Potential and Initial Momentum on the Barrier-Suppression Ionization Rate in a Circularly Polarized Laser Field

The Influence of the Effective Ionization Potential and Initial Momentum on the Barrier-Suppression Ionization Rate in a Circularly Polarized Laser Field

Floquet-engineered chiral-induced spin selectivity.

The control of electron spin, crucial to the stability of matter, offers new possibilities for manipulating the properties of molecules and materials with potential applications in spintronics and chemical reactions. Recent experiments have demonstrated that electron transmission through chiral molecules depends on the electron spin orientation, a phenomenon known as chiral-induced spin selectivity (CISS). In this study, we show that CISS can be observed in achiral systems driven by an external circularly polarized laser field in the framework of Floquet engineering. By using the Floquet theory for a time-periodically driven system to investigate spin-dependent electron transport in a two-terminal setup, we demonstrate that the spin polarization can approach unity if the light intensity is sufficiently strong, the rate of dephasing is sufficiently low, and the average chemical potential of the two leads is within an appropriate range of values, which is narrow because of the high frequency of the laser field. To obtain a broader range of energies for large spin polarization, a combination of chiral molecules and light-matter interactions is considered, and the spin polarization of electrons transported through a helical molecule driven by a laser field is evaluated.

Nonsequential double ionization of atoms driven by few-cycle counter-rotating two-color circularly polarized laser fields

Using a classical ensemble model, we investigated the dependence of nonsequential double ionization (NSDI) on the relative phase driven by few-cycle counter-rotating two-color circularly polarized (TCCP) laser fields, which consist of 1600-nm laser pulse and 800-nm laser pulse. The numerical results reveal that NSDI probability is strongly dependent on the relative phase of counter-rotating TCCP fields, which shows a sin-like behavior of period 2π. The inversion analysis shows that the returning probability of the first electron is very sensitive to the relative phase of counter-rotating TCCP fields, resulting in dependence of NSDI probability on the relative phase. Furthermore, the dynamic behavior of two electrons can be regulated by adjusting the relative phase, leading to the sensitive dependence of momentum distribution of Ar2+ ions on the relative phase of counter-rotating TCCP fields. In addition, our results show that the traveling electron can be controlled to return to the parent ion core from a relatively single direction, which leave an imprint on the momentum distribution of Ar2+ ions.

Control of the electron dynamics in solid-state high harmonic generation on ultrafast time scales by a polarization-skewed laser pulse.

Since high-order harmonic generation (HHG) from atoms depends sensitively on the polarization of the driving laser field, the polarization gating (PG) technique was developed and applied successfully to generate isolated attosecond pulses from atomic gases. The situation is, however, different in solid-state systems as it has been demonstrated that due to collisions with neighboring atomic cores of the crystal lattice strong HHG can be generated even by elliptically- and circularly-polarized laser fields. Here we apply PG to solid-state systems and find that the conventional PG technique is inefficient for the generation of isolated ultrashort harmonic pulse bursts. In contrast, we demonstrate that a polarization-skewed laser pulse is able to confine the harmonic emission to a time window of less than one-tenth of the laser cycle. This method provides a novel way to control HHG and to generate isolated attosecond pulses in solids.

Nonadiabatic Dynamics in a Continuous Circularly Polarized Laser Field with Floquet Phase-Space Surface Hopping

Nonadiabatic chemical reactions involving continuous circularly polarized light (cw CPL) have not attracted as much attention as dynamics in unpolarized/linearly polarized light. However, including circularly (in contrast to linearly) polarized light allows one to effectively introduce a complex-valued time-dependent Hamiltonian, which offers a new path for control or exploration through the introduction of Berry forces. Here, we investigate several inexpensive semiclassical approaches for modeling such nonadiabatic dynamics in the presence of a time-dependent complex-valued Hamiltonian, beginning with a straightforward instantaneous adiabatic fewest-switches surface hopping (IA-FSSH) approach (where the electronic states depend on position and time), continuing to a standard Floquet fewest switches surface hopping (F-FSSH) approach (where the electronic states depend on position and frequency), and ending with an exotic Floquet phase-space surface hopping (F-PSSH) approach (where the electronic states depend on position, frequency, and momentum). Using a set of model systems with time-dependent complex-valued Hamiltonians, we show that the Floquet phase-space adiabats are the optimal choice of basis as far as accounting for Berry phase effects and delivering accuracy. Thus, the F-PSSH algorithm sets the stage for future modeling of nonadiabatic dynamics under strong externally pumped circular polarization.

Two-body top-quark decay in a circularly polarized laser field

The top quark is distinct from all other fermions due to its high mass and short lifetime. Therefore, it decays rapidly almost exclusively into a light quark and a gauge boson. Through all of these properties, the top quark can reveal new details about the standard model and possibly about new physics. In this paper, we have investigated the two-body decay of the top quark (), where q represents one of the quarks d, s and b, in the presence of a monochromatic circularly polarized laser field. Using the lowest-order Dirac-Volkov formalism, we have analytically established the expression for the laser-assisted total decay width of the top quark. We have analyzed the dependence of the total decay width, as well as the lifetime and branching ratios (BRs) on the characteristic parameters of the laser. The results indicate that the laser has a decreasing effect on the decay width, leading to an extension of the lifetime. The lifetime behavior inside the laser field is explained by the quantum Zeno effect. More importantly, we find that the laser field with an appropriate frequency and electric field amplitude leads to a modification of the BRs either by the enhancement of the mode or by the suppression of the and modes.

Top-quark pair production by electron–positron annihilation in a circularly polarized laser field

In the framework of the electroweak standard model, we have investigated the top quarks pair production from annihilation at the energies of a future compact linear collider in the presence of an intense circularly polarized laser field. We have analytically generated the detailed expression for the laser-assisted differential cross section using the lowest-order Dirac-Volkov formalism, including the and interactions, in the center-of-mass frame. A wide range of center-of-mass energies corresponding to the future collider has been covered to study the behavior of the total cross section (TCS); this future compact linear collider constitutes an important direction in the study of top quark physics in order to improve the measurements, and the limits of some physical parameters obtained so far at the LHC and the Tevatron. We have analyzed the dependence of the laser-assisted total and partial cross sections on the energy of the center-of-mass of the colliding system and also on the parameters of the laser field, such as the number of exchanged photons, the laser field strength, and its frequency. The results indicate that the laser field decreases the order of magnitude of the TCS as much as the laser field strength increases or decreases its frequency.

Antiferromagnetic-configuration–dependent high harmonic generation in bilayer CrI3

Bilayer CrI3 accommodates both interlayer antiferromagnetic (AFM) and intralayer ferromagnetic couplings. Different alignments of intralayer ferromagnetic orders would lead to almost degenerate AFM configurations, which are insensitive to conventional techniques such as VSM and magneto-optical Kerr effect. Here, we demonstrate high harmonic generation (HHG) as a feasible means to detect the AFM configurations in bilayer CrI3 with AB and stacking orders. When the intralayer magnetic moments are aligned along the z-axis, the AB stacked bilayer CrI3 cancels the 3n-order harmonics under the circularly polarized laser field. However, the stacked bilayer contains both even and odd harmonic. The 3n-order harmonics are recovered as the intralayer magnetic moments of AB bilayer are in-plane aligned. For an in-plane linearly polarized laser field, the stacking bilayer with the magnetic moments along the x-axis contains both the even and odd harmonics in each component. However, when the magnetic moments are along the y-axis, the perpendicular component of HHG cancels out for the linearly polarized laser field along the x-axis. More interestingly, when the linearly polarized laser field is along the y-axis, the parallel component includes only the odd harmonics while the perpendicular component contains only the even harmonics. Our study provides HHG as a potential tool to detect AFM configurations.

Helicity dependent Wigner phase shift for photoionization in a circularly polarized laser field

The sensitivity of strong-field ionization to atomic orbital helicity has attracted much attention from physicists, due to its potential application in attosecond spectroscopy and spintronics. In order to intuitively observe the physical mechanisms of helicity-dependent ionization rates during photoionization, the concept of the Wigner phase can be used to characterize the different interactions between the rotating electrons and the Coulomb potential. Here, we find that in both one- and three-photon ionization schemes, the electrons liberated more easily by the circularly polarized laser field suffer less influence of the Coulomb potential during the propagation and then accumulate less Winger phase. This result indicates that the strength of the interaction between the rotating electrons and the Coulomb potential can explain the helicity-dependent ionization for different ionization mechanisms universally, which is also supported by our classical ensemble analysis. Our work provides an intuitive perspective towards the physics picture of ionization propensity rules.

Enhancement of terahertz radiation from a filament by using circularly polarized two-color laser fields

We investigate the terahertz radiation from a filament in two-color laser fields with various polarizations. It has been experimentally demonstrated that the efficiency of terahertz emission in co-rotating two-color circularly polarized laser fields is eight times as strong as that in linearly polarized two-color laser fields with parallel polarization. This enhancement is explained based on the polarization-dependent clamping intensity inside a filament and the dynamics of electrons ionizing from a single atom or molecule. Our finding provides a simple method to improve the THz intensity from an air filament in two-color fields.

Relativistic Effects for a Hydrogen Rydberg Atom in a High-Frequency Laser Field: Analytical Results

Previously published analytical results for the effects of a high-frequency laser field on hydrogen Rydberg atoms demonstrated that the unperturbed elliptical orbit of the Rydberg electron, generally is engaged simultaneously in the precession of the orbital plane about the direction of the laser field and in the precession within the orbital plane. These results were obtained while disregarding relativistic effects. In the present paper, we analyze the relativistic effect for hydrogenic Rydberg atoms or ions in a high-frequency linearly- or circularly-polarized laser field, the effect being an additional precession of the electron orbit in its own plane. For the linearly-polarized laser field, the general case, where the electron orbit is not perpendicular to the direction of the laser field, we showed that the precession of the electron orbit within its plane can vanish at some critical polar angle θc of the orbital plane. We calculated analytically the dependence of the critical angle on the angular momentum of the electron and on the parameters of the laser field. Finally, for the particular situation, where the electron orbit is perpendicular to the direction of the laser field, we demonstrated that the relativistic precession and the precession due to the laser field occur in the opposite directions. As a result, the combined effect of these two kinds of the precession is smaller than the absolute value of each of them. We showed that by varying the ratio of the laser field strength F to the square of the laser field frequency ω, one can control the precession frequency of the electron orbit and even make the precession vanish, so that the elliptical orbit of the electron would become stationary. This is a counterintuitive result.

Helicity-dependent dissociative tunneling ionization of CF4 in multicycle circularly polarized intense laser fields.

Dissociative tunneling ionization of tetrafluoromethane (CF4) in circularly polarized ultrashort intense laser fields (35 fs, 0.8 × 1014 W cm-2, 1035 nm), CF4 → CF4+ + e- → CF3+ + F + e-, has been studied by three-dimensional electron-ion coincidence momentum imaging. The photoelectron angular distribution in the recoil frame revealed that the dissociative tunneling ionization occurs efficiently when the laser electric field points from F to C. The obtained results are qualitatively consistent with the theoretical predictions by the weak-field asymptotic theory (WFAT) for tunneling ionization from the highest and next-highest occupied molecular orbitals, HOMO (1t1), and HOMO-1 (4t2), respectively. On the other hand, the angular distribution shows clear dependences on the polarization helicity, indicating that the breaking of the C-F bonds is sensitive to the helicity of the multicycle circularly polarized laser fields.

Nonsequential Double Ionization of Molecules in Counter-Rotating Two-Color Circularly Polarized Laser Fields

Nonsequential Double Ionization of Molecules in Counter-Rotating Two-Color Circularly Polarized Laser Fields

Formula omitted]-boson production via the weak process [formula omitted] in the presence of a circularly polarized laser field

In the centre of mass frame, we have studied theoretically the Z-boson resonant production in the presence of an intense laser field via the weak process e+e−→μ+μ−. Dressing the incident particles by a Circularly Polarized laser field (CP-laser field), at the first step, shows that for a given laser field’s parameters, the Z-boson cross section decreases by several orders of magnitude. We have compared the Total Cross Section (TCS) obtained by using the scattering matrix method with that given by the Breit-Wigner approach in the presence of a CP-laser field and the results are found to be very consistent. This result indicates that Breit-Wigner formula is valid not only for the laser-free process but also in the presence of a CP-laser field. The dependence of the laser-assisted differential cross section on the Centre of Mass Energy (CME) for different scattering angles proves that it reaches its maximum for small and high scattering angles. At the next step and by dressing both incident and scattered particles, we have shown that the CP-laser field largely affects the TCS, especially when its strength reaches 109Vcm−1. This result is interpreted by the fact that heavy interacting particles require high laser field intensities to affect the collision’s cross section.