Circular Dichroism in Atomic Resonance-Enhanced Few-Photon Ionization

TL;DR

This study explores how circularly polarized femtosecond pulses influence lithium atom ionization, revealing strong wavelength-dependent asymmetries and potential for polarization-based control of electron emission.

Contribution

It provides the first detailed analysis of circular dichroism effects in atomic resonance-enhanced few-photon ionization, highlighting wavelength sensitivity and resonance-driven polarization effects.

Findings

Strong wavelength-dependent circular dichroism observed.

Counter-rotating fields favor ionization near 800 nm due to resonance.

Weak intensity dependence of CD across the studied regime.

Abstract

We investigate few-photon ionization of lithium atoms prepared in the polarized 2$p$($m_\ell=\!+1$) state when subjected to femtosecond light pulses with left- or right-handed circular polarization at wavelengths between 665 nm and 920 nm. We consider whether ionization proceeds more favorably for the electric field co- or counter-rotating with the initial electronic current density. Strong asymmetries are found and quantitatively analyzed in terms of "circular dichroism" ($CD$). While the intensity dependence of the measured $CD$ values is rather weak throughout the investigated regime, a very strong sensitivity on the center wavelength of the incoming radiation is observed. While the co-rotating situation overall prevails, the counter-rotating geometry is strongly favored around 800 nm due to the 2$p$-3$s$ resonant transition, which can only be driven by counter-rotating fields. The observed features provide insights into the helicity dependence of light-atom interactions, and on the possible control of electron emission in atomic few-photon ionization by polarization-selective resonance enhancement.