Probing of Core Excitons in Solid NaF with Polarization-Selective Attosecond Time-Resolved Four-Wave Mixing Spectroscopy

TL;DR

This study uses attosecond four-wave mixing spectroscopy to investigate ultrafast decoherence of core excitons in NaF, revealing rapid decoherence due to exciton-phonon coupling and orbital angular momentum characteristics.

Contribution

It demonstrates polarization-selective attosecond spectroscopy to distinguish bright and dark core excitons and measures their ultrafast decoherence in NaF.

Findings

Core excitons decohere faster than 8 fs, limited by instrument response.

Bright core excitons have s-like orbital angular momentum.

Dark core excitons exhibit p-like angular momentum.

Abstract

Nonlinear Four-wave mixing processes are a powerful technique to unravel ultrafast dynamics in solid-state systems. Here, we employ attosecond four-wave mixing spectroscopy with one extreme ultraviolet (XUV) pump and two independently delayed, noncollinear near-infrared (NIR) probes to resolve the ultrafast decoherence of both dipole-allowed and dipole-forbidden core excitons at the Na+ L2,3 edge in sodium fluoride (NaF). The decoherence times of the core excitons are observed to be much faster than the 8 fs limit of the instrument response time, which is attributed to strong exciton-phonon coupling. Furthermore, polarization control of the NIR probes (Perpendicular and parallel polarizations) reveals that the bright core excitons exhibit s-like orbital angular momentum, while dark core excitons, reached by two-photon excitation, exhibit p-like angular momentum.