# Accurate in situ Measurement of Ellipticity Based on Sub-cycle Ionization Dynamics

## TL;DR

This paper introduces a new in situ method to accurately measure the ellipticity of elliptically polarized laser pulses by using counter-rotating pulses and analyzing ionization spectra, enhancing laser pulse characterization.

## Contribution

The paper presents a robust scheme for in situ ellipticity measurement based on phase-controlled ionization modulation, which is highly reliable and applicable across various laser parameters.

## Key findings

- Electron spectra and ion yield modulations depend on phase delay
- Method accurately determines ellipticity of EPLPs
- Applicable to a wide range of laser parameters

## Abstract

Elliptically polarized laser pulses (EPLPs) are widely applied in many fields of ultrafast sciences, but the ellipticity~($\varepsilon$) has never been in situ measured in the interaction zone of the laser focus. In this work, we propose and realize a robust scheme to retrieve the $\varepsilon$ by temporally overlapping two identical counter-rotating EPLPs. The combined linearly electric field is coherently controlled to ionize Xe atoms by varying the phase delay between the two EPLPs. The electron spectra of the above-threshold ionization and the ion yield are sensitively modulated by the phase delay. We demonstrate that these modulations can be used to accurately determine $\varepsilon$ of the EPLP. We show that the present method is highly reliable and is applicable in a wide range of laser parameters. The accurate retrieval of $\varepsilon$ offers a better characterization of a laser pulse, promising a more delicate and quantitative control of the sub-cycle dynamics in many strong field processes.