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22PHASE AND POLARIZATION PROPERTIES OF HIGH HARMONICS
GENERATED IN ELLIPTICALLY POLARIZED LASER FIELD
M. Khokhlova12, A. Gonoskov3, I. Gonoskov3, M. Ryabikin3 and V. Strelkov1
1General Physics Institute of Russian Academy of Sciences, Vavilova 38, Moscow 119991, M.V Lomonosov Moscow State University, Faculty of Physics, Leninskie Gory 1/2,
119991, Moscow, Russia 3Institute of Applied Physics of Russian Academy of Science, Ulyanov street 46, 603950, N.
Novgorod, Russia [email protected]
High-order harmonics generated in elliptically polarized laser field are elliptically polarized, and the polarization ellipse of the harmonic is rotated by a certain angle with respect to that of the fundamental (the so-called rotation angle). The origin of this rotation can be understood in the framework of the well-known "simple-man" model [1]. An explanation for the origin of the ellipticity of the harmonics generated in atomic gases by elliptically polarized laser field was offered in paper [2]. The harmonic ellipticity originates from the quantum-mechanical uncertainty of the transverse momentum of the recombining electron. Because the finite wave packet returning to the parent ion is shifted in the transverse direction (due to the ellipticity of the laser field), the parent ion "observes" a certain slope of the population density in the wave packet. This asymmetry finally leads to a certain transverse component of the dipole oscillating with a CJ2 phase shift with respect to the component directed along the momentum, and thus to the harmonic ellipticity.
We derive [3] equations describing the effect of the driving ellipticity on the HHG efficiency, harmonic ellipticity, and rotation angle for the cases of the ground state with the orbital angular momentum l=0 and l ^0. These equations provide good agreement with the results of our 3D TDSE calculations, as well as with the published experimental data. Furthermore, our study shows signi ficant enhancement of the harmonic ellipticity near the Cooper minimum. This is very important because of practical interest in the circularly polarized femto- and attosecond x-ray pulses for time-resolved diagnostics of the processes in magnetic structures. Moreover, we investigate theoretically an interesting feature which was obtained experimentally. It is known that the harmonic yield usually rapidly decreases with laser field ellipticity [4,5]. However, it was observed [6,7] that the harmonic signal sometimes behaves anomalously with the laser ellipticity, i.e. the intensity of some harmonics with the photon energy near the ionization potential rises for some range of the fundamental ellipticity; this behavior can't be described in the framework of the "simple-man" approach. We investigate this phenomenon numerically solving 3D TDSE for model argon atom in the laser field. Obtained numerical results reproduce this anomalous behavior observed in experiments.
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2. V.V. Strelkov, et al., Phys. Rev. Lett. 107, 043902 (2011).
3. V.V. Strelkov, et al., Phys. Rev. A 86, 013404 (2012).
4. Ph. Antoine, et al., Phys. Rev. A 53, 1725 (1996).
5. K.S. Budil, et al., Phys. Rev. A 48, R3437 (1993).
6. N.H. Burnett, C. Kan, and P. B. Corkum, Phys. Rev. A 51 (5), R3418 (1995).
7. M. Kakehata, et. al., Phys. Rev. A 55 (2), R861 (1997).
