Speaker: Henri Bachau, Centre des Lasers Intenses et Applications, Université de Bordeaux-CEA-CNRS, 33405 Talence Cedex

Abstract: The advent of free electron lasers delivering x-rays at high intensity opens the way to the exploration of new regimes in physics, chemistry and biology. Free electron lasers deliver now intensities of the order of 10¹⁷-10¹⁸ W/cm² [1,2,3] and even more in x-ray regime, opening the way to explore nonlinear effects at short wavelengths. The breakdown of the dipole approximation in x-ray regime has been reported in the late 1920s with its effects on photoelectron angular distributions (PADs). The nondipole (or retardation) effects arise from the terms A.P and A² in the Hamiltonian (where A denotes the field vector potential and P the momentum operator). Generally the retardation correction associated with A.P dominates, it explains in first-order the deviation of the PADs from dipole approximation [4]. Nevertheless, at high intensity, nonlinear effects (involving the emission and absorption of photons) associated with the diamagnetic term A² become important. For example, we have demonstrated that the contribution of A² dominates in stimulated Compton [5] and Raman scattering [6] in the contexts of two-color fields or one-color ultra-short pulses. During the presentation I will review these processes as well as recent work on photoionization of helium at ultra-high intensity in soft xray regime.

References:

[1] L. Young et al, Nature (London) 466, 56 (2010)

[2] G. Doumy et al, Phys. Rev. Lett. 106, 083002 (2011)

[3] H. Fukuzawa et al, Phys. Rev. Lett. 110, 173005 (2013)

[4] J.W. Cooper, Phys. Rev. A 42, 6942 (1990) and errata in Phys. Rev. A 45, 3362 (1992)

[5] H. Bachau, M. Dondera, V. Florescu, Phys. Rev. Lett. 112, 073001 (2014)

[6] H. Bachau, M. Dondera, V. Florescu, T.A. Marian, J. Phys. B: At. Mol. Phys. 50, 174003 (2017)