Speaker
Description
Atomic collective motions on the sub-10 nm length scales in condensed phase samples, including crystals, glasses, and liquids, are of significant interests for both applications including heat management in information technology, and fundamental research such as glass heat capacity anomalies. While transient grating (TG) with conventional lasers has been widely utilized to measure macroscopic heat dissipation dynamics, and EUV free electron laser (FEL) based TG has been used to measure collective excitations on the length scales of a few tens of nanometers [1], for sub-10 nm length scales, a direct observation of the atomic collective motion has been very challenging.
To resolve this technical challenge, we combine the hard X-ray split-delay optics (SDO) at the X-ray Pump Probe instrument at LCLS with total reflection mirrors and demonstrate the capability of generating TG with a period of 5 nm with 9.5 keV hard X-ray pulses. The TG signal is measured with diffuse scattering from a third X-ray pulse with the same photon energy and a controlled delay time between 0 to 14 ps with a time resolution of 10 fs. Due to the complexity of the optics, a digital twin of this setup is implemented to analyze the installation accuracy requirement, alignment procedure, and TG visibility with the simulated electric field. By adjusting optics, we perform TG measurement with periods of 5, 10, 20, and 50 nm on a series of crystalline samples, including STO and Ge in Bragg geometry. The measured signal is compared with our previous X-ray pump X-ray probe measurement on the same sample.
Reference:
[1] F. Bencivenga, et al., Sci. Adv., 5.7 (2019): eaaw5805.
