Circularly polarized attosecond pulses from molecular high-order harmonic generation by ultrashort intense bichromatic circularly and linearly polarized laser pulses

We describe the generation of high-order elliptically and circularly polarized harmonic spectra in an aligned H-2(+) molecule ion by a combination of two-colour ultrashort intense laser fields from numerical solutions of the corresponding time-dependent Schrodinger equation (TDSE). In intense bichromatic circularly and linearly or circularly polarized laser pulses with intensity I-0 and angular frequencies omega(0) and 2 omega(0), it is found that maximum molecular high-order harmonic generation (MHOHG) energies are functions of the molecular internuclear distance. Based on a classical model of laser-induced electron collisions with neighbouring ions, the optimal values of the pulse relative carrier envelope phase phi, the molecular internuclear distance R and the angle. of molecular alignment to the laser polarization axis are obtained for efficiently producing MHOHG spectra with the maximum harmonic energy I-p + 13.5U(p), where I-p is the ionization potential of the molecule and U-p = I-0/4m(e)omega(2)(0) is the ponderomotive energy of the continuum electron at intensity I-0 and frequency omega(0) of the laser pulse. The results have been confirmed from corresponding TDSE nonperturbative numerical simulations. The polarization property of the generated harmonics is also presented. The mechanism of MHOHG is further characterized with a Gabor time frequency analysis. It is confirmed that a single collision trajectory of the continuum electron with neighbouring ions dominates in the MHOHG processes. The high efficiency of the proposed MHOHG scheme provides a possible source for production of elliptically and/or circularly polarized attosecond extreme ultraviolet pulses. Circularly polarized attosecond pulses can also be generated by using intense ultrashort circularly polarized laser pulses in combination with static electric fields of comparable intensity for H-2(+) at equilibrium. A time frequency analysis also confirms the role of single recollisions as the dominant mechanism of the generation of circularly polarized harmonics.