Quantum-orbit analysis of the generation of a high-ellipticity attosecond pulse with circular-elliptical bichromatic laser fields

We study attosecond pulses synthesized from high-order harmonic generation (HHG) under different circular elliptical bichromatic laser fields by numerically solving the time-dependent Schrodinger equation. For w-3w bichromatic laser fields, the ellipticity of parts of the elliptically polarized attosecond pulse train (APT) in each optical cycle will first approach unity and then decrease as the ellipticity of the 3w field decreases. However, for the w-2w bichromatic laser field, the ellipticity of the APT remains small when the ellipticity of the 2w field decreases. Our result is well reproduced in terms of quantum-orbit theory. By analyzing the quantum orbits, the underlying physics for the generation of the nearly circularly polarized attosecond pulse with the w-3w circular elliptical bichromatic laser field is revealed. Our work provides a simple method to obtain highly elliptically polarized attosecond pulses from HHG.