Detecting optically synthesized quasi-monochromatic sub-terahertz phonon wavepackets by ultrafast x-ray diffraction

Abstract

We excite an epitaxial SrRuO3 thin film transducer by a pulse train of ultrashort laser pulses, launching coherent sound waves into the underlying SrTiO3 substrate. Synchrotron-based x-ray diffraction (XRD) data exhibiting separated sidebands to the substrate peak evidence the excitation of a quasi-monochromatic phonon wavepacket with sub-THz central frequency. The frequency and bandwidth of this sound pulse can be controlled by the optical pulse train. We compare the experimental data to combined lattice dynamics and dynamical XRD simulations to verify the coherent phonon dynamics. In addition, we observe a lifetime of 130 ps of such sub-THz phonons in accordance with the theory.

Picosecond strain dynamics inGe2Sb2Te5monitored by time-resolved x-ray diffraction

“…Figure 3 (bottom panel) shows the simulated time dependence of the GST(222) diffraction signal calculated using the one-dimensional linear chain model UDKM1DSIM. [14] The model has yielded excellent fits of ultrafast phonon dynamics in metal-insulator superlattice systems [15]. For the purpose of comparison, the model spectra plot consists of a stack of simulated spectra for the times at which the experimental data were taken, namely, −150 ps, −120 ps, −100 ps, −50 ps, −20 ps, −10 ps, 0 ps, 10 ps, 20 ps, 50 ps, 100 ps, 150 ps, 300 ps, 500 ps, 1 ns, and 10 ns.…”

Ultrafast lattice response of photoexcited thin films studied by X-ray diffraction

“…SRO proved to be an ideal transducer layer for large-amplitude and high-frequency coherent longitudinal acoustic phonons 29 due to its high damage threshold 30 and its fast electron phonon coupling time of $\approx 200$ fs (Refs. 27 and 28) resulting in a quasi-instantaneous stress after photoexcitation.…”

Time-domain sampling of x-ray pulses using an ultrafast sample response

“…7,8 On a more fundamental level, it has been shown recently that thin layer transducers can be used to excite quasi-monochromatic strain waves, which allow for studying coherent acoustic phonon dynamics. 9 In typical pump-probe measurements, a sample is excited by an ultrafast laser and subsequently probed by a time-delayed ultrashort probe pulse. For each time delay s, a snapshot of the sample is recorded.…”