X-ray induced optical reflectivity

Stephen M Durbin

doi:10.1063/1.4768803

Abstract

The change in optical reflectivity induced by intense x-ray pulses can now be used to study ultrafast many body responses in solids in the femtosecond time domain. X-ray absorption creates photoelectrons and core level holes subsequently filled by Auger or fluorescence processes, and these excitations ultimately add conduction and valence band carriers that perturb optical reflectivity. Optical absorption associated with band filling and band gap narrowing is shown to explain the basic features found in recent measurements on an insulator (silicon nitride, Si3N4), a semiconductor (gallium arsenide, GaAs), and a metal (gold, Au), obtained with ∼100 fs x-ray pulses at 500-2000 eV and probed with 800 nm laser pulses. In particular GaAs exhibits an abrupt drop in reflectivity, persisting only for a time comparable to the x-ray excitation pulse duration, consistent with prompt band gap narrowing.

Highlights

X-ray intensities at synchrotron and especially x-ray free electron laser (XFEL) sources are sufficient for absorption and subsequent de-excitation processes to significantly perturb the physical properties of the target material
The increase in conduction electrons and valence holes causes an abrupt change in optical reflectivity, which makes the x-ray pump optical probe technique a valuable tool for characterizing the dynamical response of the material as well as the temporal characteristics of the incident x-ray pulse.[5,6]
Changes in optical reflectivity are directly dependent on changes in absorption: when absorption increases at the optical probe energy, for example, fewer photons can be reflected

Summary

Introduction

X-ray intensities at synchrotron and especially x-ray free electron laser (XFEL) sources are sufficient for absorption and subsequent de-excitation processes to significantly perturb the physical properties of the target material. This applies to insulators and semiconductors where x-ray absorption has produced a significant density of excited electrons in the conduction band.

Results

Conclusion