All-optical birth time measurement of circularly polarized attosecond pulses

The advancement of attosecond science has made circularly polarized attosecond pulses increasingly essential. Despite ongoing research on their generation, few studies have explored methods of characterizing these pulses. To address this issue, we propose an all-optical technique for measuring the birth time of circularly polarized attosecond pulses generated by crossing two counter-rotating circularly polarized pulses in a noncollinear geometry. By introducing a weak second-harmonic to alter the phase of the electron trajectories, the even and odd high harmonics (HHs) are separated spatially, allowing one to determine the birth time of the attosecond pulses from the modulation of the even harmonics as a function of delay between the perturbing and driving fields. The spatial isolation of the even harmonic allows extending the method to the characterization of isolated attosecond pulse, which is challenging for existing in-situ methods. It provides deeper insight into the control and generation of circularly polarized HHs in both time and space domains, which will benefit circular dichroism attosecond metrology.