Amplitude modulation effect on in-situ temporal characterization of high harmonic attosecond pulses

Ultrafast pulse characterization is one of the fundamental techniques in ultrafast sciences, including the characterization of high harmonic attosecond pulses. The in-situ measurement techniques enable all-optical, spatiotemporal determination of the atto-chirp by applying a perturbing laser field, which is believed to introduce only phase modulations to attosecond pulses. In this paper, we have experimentally revisited in-situ measurement techniques with a collinear or an oblique second-harmonic perturbing laser field. Reduced oscillation contrast ratios of even-order harmonic intensities are observed in the collinear experiment, and the far-field angular profiles of even-order harmonics vary with the perturbing laser phase delay in the oblique one. Both observations confirm the amplitude modulation effect on the in-situ temporal measurement technique of high harmonic attosecond pulses, and such an effect is due to the perturbing laser-induced photoionization rate variations. Finally, we incorporate the amplitude modulation effect into in-situ measurements by utilizing the oscillation contrast ratio information and assuming a constant amplitude-phase modulation delay, correcting the underestimated atto-chirps.