Accurate in situ Measurement of Ellipticity Based on Subcycle Ionization Dynamics

Elliptically polarized laser pulses (EPLPs) are widely applied in many fields of ultrafast sciences, but the ellipticity ($ϵ$) has never been in situ measured in the interaction zone of the laser focus. In this Letter, we propose and realize a robust scheme to retrieve the $ϵ$ by temporally overlapping two identical counterrotating 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 $ϵ$ 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 $ϵ$ offers a better characterization of a laser pulse, promising a more delicate and quantitative control of the subcycle dynamics in many strong field processes.

Figure 1

Schematic diagram of the experimental setup. The laser propagates along the $x$ axis and the polarization plane of each near-circular pulse lies in the $y\text{−}z$ plane. The insert shows the temporal overlap between the two counterrotating pulses and the combined linearly polarized field with the rotation of its polarization and variation in the peak field strength for different phase delay $\mathrm{\Delta }\phi$.

Figure 2

The phase-delay dependence of the ATI electrons and the ion yield. (a) The energy-integrated angular distributions at various delays (symbols for the measurements and lines for the TDSE simulations). (b) The experimentally measured angle-integrated ATI spectra at different delays, compared against the TDSE calculations under the same laser parameters shown in (d). In (a), (b), and (d), the curve for each delay is relatively shifted vertically for a better visibility. In (c), the normalized ion yield for the single ionization of Xe is shown at different phase delays from the measurements (squares) and the PPT calculations (solid line).

Figure 3

The normalized ion yield as a function of the phase delay for (a) different laser intensities (in $\mathrm{TW}/{\mathrm{cm}}^2$) at a fixed $ϵ=0.87$; (b) various values of the ellipticity at a fixed laser intensity of $200\mathrm{TW}/{\mathrm{cm}}^2$. For every case, the lines stand for the theoretical calculations, while the symbols for the experimental measurements where the value of the ellipticity is estimated by the usual optical method.

Figure 4

The modulation $R$ at different laser parameters. (a) and (b) show the dependence of the laser intensity at various $ϵ$. Also shown in (a) as symbols are the available experimental data. (c) shows the pulse duration dependence for two different values of $ϵ$. (d) shows the wavelength dependence for $ϵ=0.87$.