Resonant Self-Diffraction of Femtosecond Extreme Ultraviolet Pulses in Cobalt

TL;DR

This paper demonstrates resonant self-diffraction of femtosecond EUV pulses in cobalt, revealing spectral features near the absorption edge and proposing a new spectroscopy technique for EUV coherent effects.

Contribution

It introduces the first observation of EUV self-diffraction in a metal, linking it to electronic temperature changes and suggesting its use for EUV spectroscopy.

Findings

Sharp diffraction peak at cobalt's M2,3 absorption edge at 59 eV

Detection of fine spectral structure above the absorption edge

Theoretical model correlates diffraction efficiency with electronic temperature

Abstract

Self-diffraction is a non-collinear four-wave mixing technique well-known in optics. We explore self-diffraction in the extreme ultraviolet (EUV) range, taking advantage of intense femtosecond EUV pulses produced by a free electron laser. Two pulses are crossed in a thin cobalt film and their interference results in a spatially periodic electronic excitation. The diffraction of one of the same pulses by the associated refractive index modulation is measured as a function of the EUV wavelength. A sharp peak in the self-diffraction efficiency is observed at the M$_{2,3}$ absorption edge of cobalt at 59 eV and a fine structure is found above the edge. The results are compared with a theoretical model assuming that the excitation results in an increase of the electronic temperature. EUV self-diffraction offers a potentially useful spectroscopy tool and will be instrumental in studying coherent effects in the EUV range.