Phase- and intensity-resolved measurements of above threshold ionization by few-cycle pulses

TL;DR

This study measures how the carrier-envelope phase and intensity of few-cycle laser pulses affect the momentum distribution of electrons from above-threshold ionization of argon, revealing phase-dependent asymmetries and interference patterns.

Contribution

It provides the first detailed phase- and intensity-resolved measurements of above-threshold ionization with few-cycle pulses, validating theoretical models and offering a new method to determine laser intensity.

Findings

Maximum phase asymmetry at 2 U_P persists across intensities

Local maxima at 0.3 and 0.8 U_P observed

Asymmetry amplitude correlates with peak laser intensity

Abstract

We investigate the carrier-envelope phase and intensity dependence of the longitudinal momentum distribution of photoelectrons resulting from above-threshold ionization of argon by few-cycle laser pulses. The intensity of the pulses with a center wavelength of 750\,nm is varied in a range between $0.7 \times 10^{14}$ and $\unit[5.5 \times 10^{14}]{W/cm^2}$. Our measurements reveal a prominent maximum in the carrier-envelope phase-dependent asymmetry at photoelectron energies of 2\,$U_\mathrm{P}$ ($U_\mathrm{P}$ being the ponderomotive potential), that is persistent over the entire intensity range. Further local maxima are observed at 0.3 and 0.8\,$U_\mathrm{P}$. The experimental results are in good agreement with theoretical results obtained by solving the three-dimensional time-dependent Schr\"{o}dinger equation (3D TDSE). We show that for few-cycle pulses, the carrier-envelope phase-dependent asymmetry amplitude provides a reliable measure for the peak intensity on target. Moreover, the measured asymmetry amplitude exhibits an intensity-dependent interference structure at low photoelectron energy, which could be used to benchmark model potentials for complex atoms.