Tunneling electron-positron pair production is studied in a new setup in which a strong low-frequency and a weak high-frequency laser field propagate in the same direction and collide head-on with a relativistic nucleus. The electron-positron pair-production rate is calculated analytically in the limit in which in the nucleus rest frame, the strong field is undercritical and the frequency of the weak field is below and close to the pair-production threshold. By changing the frequency of the weak field, one can reduce the tunneling barrier substantially. As a result, tunneling pair production is shown to be observable with presently available technology.

Laser filamentation generated when intense laser pulses propagate in air has been an attractive phenomenon having a variety of potential applications such as detection and spectroscopy of gases at far distant places. It was discovered recently that the filamentation in air induces ‘lasing', showing that electronically excited N2+ is population-inverted, exhibiting marked contrast to the common understanding that molecular ions generated by intense laser fields are prepared mostly in their electronic ground states. Here, to clarify the mechanism of the population inversion, we adopt few-cycle laser pulses, and experimentally demonstrate that the lasing at 391 nm occurs instantaneously after N2+ is produced. Numerical simulations clarify that the population inversion is realized by the post-ionization couplings among the lowest three electronic states of N2+. Our results shed light on the controversy over the mechanism of the air lasing, and show that this post-ionization coupling can be a general mechanism of the atmospheric lasing.

The emission of a pair of entangled photons by an electron in an intense laser field can be described by two-photon transitions of laser-dressed, relativistic Dirac-Volkov states. In the limit of a small laser field intensity, the two-photon transition amplitude approaches the result predicted by double Compton scattering theory. Multi-exchange processes with the laser field, including a large number of exchanged laser photons, cannot be described without the fully relativistic Dirac-Volkov propagator. The nonperturbative treatment significantly alters theoretical predictions for future experiments of this kind. We quantify the degree of polarization correlation of the photons in the final state by employing the well-established concurrence as a measure of the entanglement.PACS numbers: 12.20. Ds, 34.50.Rk, 32.80.Wr, 03.65.Ud, 13.60.Fz Introduction.-In ordinary Compton scattering [1], a photon is scattered inelastically by an electron. For photons with energy much less than the electron's rest mass, the quantum mechanical expression for the cross section agrees with the one obtained by classical electrodynamics. Nonlinear Compton scattering is encountered when several photons from a strong laser beam are scattered by a free electron to produce a photon of different energy; this process has been calculated theoretically [2,3] and successfully measured [4,5]. Recently, there has been an increased interest in a different nonlinear generalization of Compton scattering where a free electron collides with a laser pulse and emits two photons at the same time. This process has no classical counterpart, and indeed, as we will see, the two photons exhibit a paradigmatic quantum feature: namely, their polarizations are entangled. Properly optimized, two-photon emission from backscattering of laser photons at an electron beam holds the promise of providing entangled light at much larger energy than conventionally used for quantum information purposes [6].With relativistically strong lasers being available in many laboratories worldwide, the current record being a laser intensity of 10 22 W/cm 2 at the focus [7], the quest for observing genuine laser-induced quantum effects in the relativistic regime continues. However, the peak field strengths are still orders of magnitudes below the quantum electrodynamic (QED) critical field E c = −m 2 /e = 10 16 V/cm for pair creation (here m and e = −|e| denote the mass and charge of the electron, respectively, and we use natural relativistic units c = = ǫ 0 = 1). Two-photon emission by a laser-dressed electron via nonperturbative double Compton backscattering is a strong-field, relativistic quantum effect which could be observed without the additional complications connected with the ultra-relativistic particle beams necessary for laser-dressed pair creation [8,9,10].The theory of perturbative double Compton scattering, the reaction in which one photon scatters on an electron to produce two final photons was calculated by Mandl and Skyrme [11], recently reexamined in [12], and exper...

We experimentally investigate the dependence of the fragmentation behavior of the ethylene dication on the intensity and duration of the laser pulses that initiate the fragmentation dynamics by strong-field double ionization. Using coincidence momentum imaging for the detection of ionic fragments, we disentangle the different contributions of ionization from lower-valence orbitals and field-driven excitation dynamics to the population of certain dissociative excited ionic states that are connected to one of several possible fragmentation pathways towards a given set of fragment ions. We find that the excitation probability to a particular excited state and therewith the outcome of the fragmentation reaction strongly depend on the parameters of the laser pulse. This, in turn, opens up new possibilities for controlling the outcome of fragmentation reactions of polyatomic molecules in that it may allow one to selectively enhance or suppress individual fragmentation channels, which was not possible in previous attempts of controlling fragmentation processes of polyatomic molecules with strong laser fields.

In the relativistic and the nonrelativistic theoretical treatment of moderate and high-power lasermatter interaction, the generalized Bessel function occurs naturally when a Schrödinger-Volkov and Dirac-Volkov solution is expanded into plane waves. For the evaluation of cross sections of quantum electrodynamic processes in a linearly polarized laser field, it is often necessary to evaluate large arrays of generalized Bessel functions, of arbitrary index but with fixed arguments. We show that the generalized Bessel function can be evaluated, in a numerically stable way, by utilizing a recurrence relation and a normalization condition only, without having to compute any initial value. We demonstrate the utility of the method by illustrating the quantum-classical correspondence of the Dirac-Volkov solutions via numerical calculations.

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We experimentally investigate the ionization mechanism behind the formation of remarkably high charge states observed in the laser-pulse-induced fragmentation of different hydrocarbon molecules by Roither et al. [Phys. Rev. Lett. 106, 163001 (2011)], who suggested enhanced ionization occurring at multiple C-H bonds as the underlying ionization mechanism. Using multiparticle coincidence momentum imaging we measure the yield of multiply charged fragmenting ethylene and acetylene molecules at several intensities and pulse durations ranging from the few-cycle regime to 25 fs. We observe, at constant intensity, a strong increase of the proton energy with increasing laser pulse duration. It is shown that this is caused by a strong increase in the yield of highly charged parent molecular ions with pulse duration. Based on experimental evidence we explain this increase by the necessary population of precursor states in the parent ion that feature fast C-H stretch dynamics to the critical internuclear distance, where efficient ionization via enhanced ionization takes place. For increasing pulse duration these precursor ionic states are more efficiently populated, which leads in turn to a higher enhanced-ionization probability for longer pulses. Our work provides experimental evidence for the existence of a multiple-bond version of enhanced ionization in polyatomic molecules.