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Abstract

High-order harmonic generation in two-color (omega - 2 omega) counter-rotating circularly polarized laser fields opens the path to generate isolated attosecond pulses and attosecond pulse trains with controlled ellipticity. The generated harmonics have alternating helicity, and the ellipticity of the generated attosecond pulse depends sensitively on the relative intensities of two adjacent, counter-rotating harmonic lines. For the s-type ground state, such as in helium, the successive harmonics have nearly equal amplitude, yielding isolated attosecond pulses and attosecond pulse trains with linear polarization, rotated by 120 degrees from pulse to pulse. In this work, we suggest a solution to overcome the limitation associated with the s-type ground state. It is based on modifying the three propensity rules associated with the three steps of the harmonic generation process: ionization, propagation, and recombination. We control the first step by seeding high-order harmonic generation with XUV light tuned well below the ionization threshold, which generates virtual excitations with the angular momentum corotating with the omega field. We control the propagation step by increasing the intensity of the omega field relative to the 2 omega field, further enhancing the chance of the omega field being absorbed versus the 2 omega field, thus favoring the emission corotating with the seed and the omega field. We demonstrate our proposed control scheme using a helium atom as a target and solving a time-dependent Schrodinger equation in two and three dimensions.