Figure 1

Schematic diagram displaying positronium dynamics in an intense laser field. The bound system depicted by the density of its wave function may ionize in the laser field. Once free, both electron $e^{-}$ and positron $e^{+}$ could be described as classical particles. Their trajectories are shown by the solid lines. The electric field $\mathbf{E}$ accelerates both particles in opposite directions, while the Lorentz force due to the magnetic field $\mathbf{H}$ leads to an identical drift in the propagation direction ${\mathbf{k}}_{\mathbf{L}}$. Without an initial center-of-mass motion, these trajectories are symmetric and thus both particles overlap periodically, giving rise to possible recombinations and thus high-frequency light emission.Reuse & Permissions

Figure 2

Differential high-harmonic spectral intensities $(d{I}_n/d\Omega ){\mathrm{s}\mathrm{i}\mathrm{n}}^{-2}\theta$ at odd harmonic numbers $n$ as a function of the photon energy of the respective harmonic with $\theta$ being the harmonic emission angle with respect to the polarization direction. (a) The spectrum of a single positronium with $\sim 4.1\times {10}^6$ harmonics via a coherent field of intensity $I=8.63\times {10}^{12}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ at a wavelength of $\lambda =100\mu \mathrm{m}$ close to that provided by the Centre Laser Infra-rouge Orsay free-electron laser [15], $\xi =0.25$ and $\mathbf{B}=0$. (b) The spectrum of a ${\mathrm{N}\mathrm{e}}^{7+}$ ion in a yttrium aluminum garnet laser field with $\lambda =1054\mathrm{n}\mathrm{m}$ at $I=1.5\times {10}^{17}\mathrm{W}/{\mathrm{c}\mathrm{m}}^2$ ($\xi =0.35$). At those parameters both systems yield approximately the same cutoff frequency, although with a yield in the spectral intensity of the cutoff harmonics differing by about 15 orders of magnitude.Reuse & Permissions

Figure 3

Feynman diagrams displaying $\gamma$-photon emission via electron-positron annihilation by (a) multiphoton annihilation in superstrong laser fields at $\xi \gtrsim 100$ with emission of $n\gtrsim {10}^6$ laser photons along with one $\gamma$ quantum; (b) annihilation in a laser field at $\xi \ll 100$ with emission of laser photons along with two $\gamma$ quanta; (c) annihilation of ortho-positronium with emission of three photons without laser field. Bold lines correspond to the electron (positron) Volkov states, i.e., involving a laser field; dashed lines correspond to the emitted photons.Reuse & Permissions