Suo Tang

We investigate the multiphoton pair production in circularly polarized field via two level model. There appears obvious discrete ring structures in the momentum distribution of the created particles, in which the ring radius is mainly controlled by the number of the photons absorbed in the creation with the energy conservation and could also be modulated by the spin of the created pair. These multiphoton rings become narrower when both of the pair particles' spin are aligned with the direction of the field rotation, and become broader if both spin are antiparallel to that direction. This spin-modulation can be simply understood with the angular momentum conservation, as less orbital angular momentum from the absorbed photons would be transferred to the created particles if their spins are aligned with the field rotation. The orbital angular momentum of the created particles is manifested as the vortex structure in the phase of the momentum distribution, and valued as the topological charge of this phase vortex. We also study the spin entanglement between the created particles, and reveal that the entanglement becomes stronger with the increase of the particles' transverse momentum, and gets sharp peak in the transition between different multiphoton rings, where the topological charge of the phase vortex is changed.

We investigate the phase interference effects in the nonlinear Compton scattering via the collision between a high-energy electron and the laser in the intermediate intensity region, and reveal that the importance of interference effects on the scattered photons depends sensitively on the relative polarization of the scattered photons and laser pulse. For scattered photons polarized parallel to the laser field, the effective distance between two interfered phase points is much larger than that for perpendicularly polarized photons. To signalize the phase interference effect in potential experiments, we introduce the double-pulse scenario for the scattering process, and show that the interference between two phase-separated pulses could significantly modulate the energy-momentum distribution of the scattered photons, leading to the reshaping of the scattered photons’ energy spectrum. By narrowing the angular acceptance of the scattered photons, the spectral modulation can be amplified and sustained for an experiment-realizable phase separation between two pulses.

We investigate the spin correlations between electron-positron pairs created from a photon when it scatters in a high-intensity laser pulse via the nonlinear Breit-Wheeler process. We find that the spin states of the generated electron-positron pair can exhibit strong entanglement, with the degree being sensitive to the photon energy, laser intensity, and the relative polarization of the photon and laser pulse. Photons with a high degree of polarization can create strongly entangled pairs even in the intermediate intensity (nonperturbative) regime. We find that if the photons are provided by a Compton source, strongly spin-entangled electron-positron pairs can be generated with technology available today.

We investigate the phase interference effects in the nonlinear Compton scattering via the collision between an high-energy electron and the laser in the intermediate intensity region, and reveal that the importance of interference effects on the scattered photons depends sensitively on the relative polarization of the scattered photons and laser pulse. For scattered photons polarized parallel to the laser field, the effective distance between two interfered phase points is much larger than that for perpendicularly polarized photons. To signalize the phase interference effect in potential experiments, we introduce the double-pulse scenario for the scattering process, and show that the interference between two phase-separated pulse could significantly modulate the energy-momentum distribution of the scattered photons, leading to the reshaping of the scattered photons' energy spectrum. By narrowing the angular acceptance of the scattered photons, the spectral modulation can be amplified and sustained for an experiment-realizable phase separation between two pulses.

We investigate the nonlinear Compton photon source for upcoming laser-particle experiments in the collision scenario of high-energy electron beams and relativistic laser pulses. The stronger laser field could not only improve the scattering probability but also induce broader photon beam divergence. To maximize the photon flux in a realistic narrow angular acceptance, we show that a balance must be met for the laser intensity to boost the scattering probability and simultaneously confine its beam divergence. For the currently experiment-concerned 10 GeV energy regime, the balanced laser intensity locates in the intermediate intensity regime of ξ ∼ O ( 1 ) . In these regimes of laser intensity and electron energy, the accepted photons within a relatively narrow acceptance are highly polarized in the state closely relating to the polarization of the laser pulse, which actually implies a potential route to finely control the photon polarization via the laser polarization. Published by the American Physical Society 2025

The vortex state properties of a pair of scalar particles produced in a strong circularly polarized electromagnetic standing wave field are investigated under the dipole approximation in the multiphoton regime. We find that vortex orientation is related to the intrinsic orbital angular momentum of created particles associating with the helicity of absorbed photons, while the magnitude of the orbital angular momentum, i.e., the topology charge, is determined by the number of absorbed photons. Moreover, the properties of particle creation and vortices formation can be understood by analyzing the pair production process in quasiparticle representation. This study provides new insights into the angular momentum transfer from field to particle in the scalar pair production process in vacuum. It is expected that there are similar findings about vortex features for different spin alignment in electron-positron pair production in strong fields via the topology charge as a new freedom.

We have formulated a generalized two level model for studying the vacuum electron-positron pair production in strong classical background fields via slowly varying envelope approximation. It can provide momentum spectra with fully spin resolved components for all possible combined spin states of the particle and antiparticle simultaneously in multidimensional time-dependent electric fields. Moreover, we have also investigated the validity of the two level model for fermions (scalar particles) by comparing the results with those by equal-time Dirac-Heisenberg-Wigner (Feshbach-Villars-Heisenberg-Wigner) formalism in different regimes of pair creation, i.e., multiphoton and tunneling dominated mechanisms. It is found that the results are consistent with each other, indicating the good approximation of the two level model. In particular, in terms of the two level model, we found that the contribution of the particle momentum spectra is the greatest when the spin states of the particle and antiparticle are parallel with S = 1 . It is believed that by this two level model one can extend researches on pair production for more different background fields, such as a slowly varying spatial-temporal one. Significantly, we demonstrate that the generalized two level model is equivalent to the spin resolved quantum kinetic theory. In comparison to the equal-time Dirac-Heisenberg-Wigner and Feshbach-Villars-Heisenberg-Wigner methods, the two-level model is simpler and has the potential to deliver faster computational performance.