X-ray pump optical probe cross-correlation study of GaAs

Area of Science:

• Condensed matter physics
• Materials science
• Ultrafast spectroscopy

Background:

• Optical-pump, X-ray probe techniques are standard for studying ultrafast dynamics.
• Advancements in synchrotron and X-ray free-electron laser (XFEL) capabilities open new research avenues.
• The reverse approach, X-ray pump followed by optical probe, is less explored.

Purpose of the Study:

• To investigate the ultrafast optical response of semiconductors to X-ray excitation.
• To explore the feasibility of using X-ray-induced changes in optical properties for time-resolved measurements.
• To develop novel cross-correlation techniques for X-ray sources.

Main Methods:

• Utilizing an X-ray pump beam to excite a thin gallium arsenide (GaAs) sample.
• Employing an optical probe beam to measure changes in the sample's transmission spectrum.
• Investigating the effects at photon energies above and below the GaAs bandgap.

Main Results:

• An X-ray pump induced a transformation of the GaAs sample from a strong absorber to a nearly transparent window for photon energies above the bandgap within 100 picoseconds.
• The opposite effect, X-ray induced optical opacity, was observed for photon energies below the bandgap.
• These findings highlight the complex ultrafast many-body response of semiconductors to X-ray absorption.

Conclusions:

• The study demonstrates a novel X-ray pump-optical probe technique for studying ultrafast dynamics in semiconductors.
• This method provides a new approach for X-ray/optical cross-correlation measurements at synchrotrons and XFELs.
• The observed phenomena offer insights into the fundamental ultrafast response of materials to intense X-ray irradiation.