Control of helicity of high-harmonic radiation using bichromatic circularly polarized laser fields
High-harmonic generation in two-colour ($ω-2ω$) counter-rotating circularly polarised 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 polarisation, rotated by 120$^{\circ}$ 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: ionisation, propagation, and recombination. We control the first step by seeding high harmonic generation with XUV light tuned well below the ionisation threshold, which generates virtual excitations with the angular momentum co-rotating with the $ω$-field. We control the propagation step by increasing the intensity of the $ω$-field relative to the $2ω$-field, further enhancing the chance of the $ω$-field being absorbed versus the $2ω$-field, thus favouring the emission co-rotating with the seed and the $ω-$field. We demonstrate our proposed control scheme using Helium atom as a target and solving time-dependent Schr{ö}dinger equation in two and three-dimensions.