Origin for Ellipticity of High-Order Harmonics Generated in Atomic Gases and the Sublaser-Cycle Evolution of Harmonic Polarization

Abstract

We investigate numerically and analytically the polarization properties of high-order harmonics generated by an atom in intense elliptically polarized laser field. The offset angle of the harmonic polarization ellipse can be well described with the semiclassic "simple-man" high-harmonic generation model. The harmonic ellipticity itself, however, can be hardly understood within this model. We show that this ellipticity originates from quantum-mechanical uncertainty of the electron motion. We develop a theoretical approach describing this ellipticity and, more generally, the time evolution of the high-harmonic polarization state within the laser cycle. The analytical results are verified with the exact numerical solution; to make the comparison accurately, we develop a specific technique for separating the contributions of quantum paths in the numerical calculation.

Ellipticity of odd-even harmonics from oriented asymmetric molecules in strong linearly polarized laser fields

“…The interference of the long and short electronic trajectories [39] can significantly influence the ellipticity [15]. To exclude this potential influence, in the following simulations, we focus on the contribution of the short trajectory which dominates experimental HHG [40,41], This is achieved by putting the absorbing boundary along the x axis at xo = 1.2£/u;jj, similar to the procedure used in Ref.…”

“…Experimental and theoretical studies show that the elliptical polarization state of harmonics can be generated from aligned molecules irradiated by a linearly polarized laser field. The possible origin of the ellipticity of HHG has been attributed to the multielectron effect [11], the Coulomb effect [13,14], twocenter interference [10], and quantum-mechanical uncertainty of the electron motion [15], As this polarization of HHG is closely related to not only the parallel harmonics and the harmonics emitted perpendicular to the laser polarization (i.e., the perpendicular harmonics) but also the phase of the harmonics, it provides other deep insights into the mechanism of the molecular HHG [16].…”

High harmonic generation with fully tunable polarization by train of linearly polarized pulses

“…The linear trajectory and 'head on' recollision yield bright linearly polarized high harmonic radiation. Introducing ellipticity to the laser field can result in elliptically polarized harmonics [10][11][12]. However, in this case the high harmonic efficiency drops drastically as a result of the electron trajectory, which misses the parent ion upon its return [13,14].…”

High Harmonics with Controllable Polarization by a Burst of Linearly-Polarized Driver Pulses

“…For example, dynamical symmetries (DSs) present in the laser field can impose selection rules on the harmonic emission [5,6]. Additionally, if the driver is linearly-polarized, then so are the emitted harmonics, while an elliptically polarized driver leads to elliptical harmonics [7][8][9]. Unfortunately, an elliptical driver drastically reduces the probability for re-collisions, lowering the yield [10,11].…”