Strong-field Breit-Wheeler pair production in short laser pulses: Identifying multiphoton interference and carrier-envelope-phase effects

The creation of electron-positron pairs by the strong-field Breit-Wheeler process in intense short laser pulses is investigated in the framework of laser-dressed quantum electrodynamics. Regarding laser field parameters in the multiphoton regime, special attention is brought to the energy spectrum of the created particles, which can be reproduced and explained by means of an intuitive model. The model is based on the probabilities of multiphoton events driven by the spectral components of the laser pulse. It allows us, in particular, to identify interferences between different pair production channels which exhibit a characteristic dependence on the laser carrier-envelope phase.

Figure 1

Energy spectrum of positrons $dP=\frac{d^{3}P}{d{E}_{p_{+}}{d}^2{\mathrm{\Omega }}_{e^{+}}}\frac{\partial E_{p_{+}}}{\partial E_L}(E_L)$ (black line) in units of $1/m$ as a function of the required laser photon energy $E_L$ in units of the central laser frequency ${\omega }_c$. The top axis shows the corresponding positron energy. Colored lines depict model estimates for partial probabilities induced by the absorption of different numbers $N$ of laser photons as indicated in the legend and by the symbols $[N]$. These positrons with ${\varphi }_{e^{+}}=\frac{\pi }{4}$, ${\theta }_{e^{+}}=0.3\pi$ result from the head-on collision of a laser pulse with ${\xi }_{\max }=0.1$, $N_{\mathrm{osc}}=6$, ${\omega }_c=0.9m$ and $\chi =0$ and a gamma quantum of energy ${\omega }_{\gamma }=3.015m$.

Figure 2

Positron energy spectrum $dP$ of Eq. (31) (black line) in units of $1/m$ obtained for ${\omega }_c=3.6m$ and ${\xi }_{\max }=0.5$ in order to demonstrate emission processes at low energies. Remaining parameters are identical to Fig. 1. The colored lines depict model estimates for processes involving a total number $N$ of laser photons, where $N$ is indicated in the legend and by the labels $[N]$. The solid lines correspond to absorption-only processes, while the symbol lines show processes where at least one photon is emitted into the laser wave. The model expressions for the three- and four-photon absorption-only processes are too small to be seen here. Subticks indicate laser photon energies $E_L$ which correspond to integer multiples of the laser basis frequency ${\omega }_b$.

Figure 3

Positron energy spectra $dP$ in units of $1/m$ for various values of the carrier-envelope phase as indicated in the legend and for ${\xi }_{\max }=0.05$ (left), ${\xi }_{\max }=0.1$ (center) and ${\xi }_{\max }=0.2$ (right). Remaining parameters are the same as in Fig. 1. The inlets show the respective last multiphoton peak before the one-photon tail begins. Subticks indicate laser photon energies $E_L$ which correspond to integer multiples of the laser basis frequency ${\omega }_b$.