Transient diffuse scattering in GeSbTe (GST) as a timing tool for ultrafast X-ray diffraction

Abstract

Precise determination of the time of simultaneous incidence of laser pump and X-ray probe pulses is essential for ultrafast X-ray diffraction experiments. This work investigates the use of diffuse scattering from the phase-change material Ge2Sb2Te5(GST) as a timing tool for pump-probe measurements at the FemtoMAX beamline at MAX IV Laboratory.

A complete data analysis pipeline was developed to process time-resolved powder diffraction data. Individual detector images were sorted into pump-probe delay bins using the jitter monitors at FemtoMAX, azimuthally integrated using the pyFAI software package, and combined to produce time-resolved scattering profiles.

Measurements obtained during three experimental campaigns reproduced the characteristic ultrafast increase in diffuse scattering intensity following femtosecond laser excitation of GST. By combining data from multiple delay scans, the intensity increase was resolved to occur within 70 fs, consistent with previous measurements at FemtoMAX. The impact of laser fluence and sample temperature on the diffuse scattering response was also investigated, with the signal becoming difficult to resolve at low excitation fluences.

These results demonstrate that transient diffuse scattering from GST provides a reproducible marker of time zero for ultrafast X-ray diffraction experiments in a variety of experimental conditions. The analysis framework developed in this work provides a reliable methodology for processing future pump-probe diffraction measurements, and supports the continued use of GST as a time-zero monitor at FemtoMAX.

Popular Abstract

Ultrafast X-ray diffraction is a powerful technique for observing how materials change on extremely short timescales. By using a short laser pulse to excite a material and an X-ray pulse to measure it, scientists can measure structural changes occurring within trillionths of a second. To interpret these measurements, it is important to know exactly when the laser and X-ray pulses hit the sample simultaneously. This moment is known as time zero.

This thesis investigates whether the material germanium-antimony-tellurium (GST) can be used as a reliable time-zero monitor at the FemtoMAX beamline at MAX IV Laboratory in Lund. GST is a phase-change material that exhibits a rapid change in its X-ray scattering after being excited by a laser pulse. This fast signal can then be used to pinpoint time zero.

A data-analysis pipeline was developed to process thousands of individual X-ray detector images and sort them according to the timing delay between the pulses. The images were then analysed to track changes in the X-ray signal following laser excitation. Measurements from several experimental campaigns reproduced a rapid increase in scattering, as was previously observed in GST. The response was investigated under different laser fluences, sample compositions, temperatures and X-ray incidence angles.

The results show that GST produces a reproducible, and sufficiently fast response to be used as a time-zero monitor for ultrafast X-ray diffraction experiments at FemtoMAX.

• author: Dolan, Sally
• supervisor: Jörgen Larsson
• organization: Atomic Physics; Department of Physics
• alternative title: Transient diffus spridning från GeSbTe (GST) som ett verktyg för tidsbestämning vid ultrasnabb röntgendiffraktion
• course: PHYM03 20261
• year: 2026
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: X-ray diffraction, femtosecond X-ray, femtosecond laser, FemtoMAX, time zero, GST, germanium-antimony-tellurium, pump-probe experiments, ultrafast
• publication/series: LRAP, Lund Reports on Atomic Physics
• report number: LRAP 632
• language: English
• id: 9249042
• date added to LUP: 2026-08-26 08:23:01
• date last changed: 2026-08-26 08:23:01