Real-time observation of coherent acoustic phonons generated by an acoustically mismatched optoacoustic transducer using x-ray diffraction

Abstract

The spectrum of laser-generated acoustic phonons in indium antimonide coated with a thin nickel film has been studied using time-resolved x-ray diffraction. Strain pulses that can be considered to be built up from coherent phonons were generated in the nickel film by absorption of short laser pulses. Acoustic reflections at the Ni–InSb interface leads to interference that strongly modifies the resulting phonon spectrum. The study was performed with high momentum transfer resolution together with high time resolution. This was achieved by using a third-generation synchrotron radiation source that provided a high-brightness beam and an ultrafast x-ray streak camera to obtain a temporal resolution of 10 ps. We also carried out simulations, using commercial finite element software packages and on-line dynamic diffraction tools. Using these tools, it is possible to calculate the time-resolved x-ray reflectivity from these complicated strain shapes. The acoustic pulses have a peak strain amplitude close to 1%, and we investigated the possibility to use this device as an x-ray switch. At a bright source optimized for hard x-ray generation, the low reflectivity may be an acceptable trade-off to obtain a pulse duration that is more than an order of magnitude shorter.

X-ray free-electron laser based dark-field X-ray microscopy: a simulation-based study

“…An optical-laser-pump-X-ray-probe scheme has previously been applied to study phonon dynamics using diffraction methods at XFELs, where the enhanced brightness and pulse duration allow for the finer time resolutions necessary to study phonon dynamics (Lindenberg et al, 2000;Larsson et al, 2002;Persson et al, 2015;Jarnac et al, 2017;Lemke et al, 2018). Here, we propose to implement DFXM at an XFEL using such a pump-probe scheme.…”

Tracking picosecond strain pulses in heterostructures that exhibit giant magnetostriction

“…Multiple reflections of the strain pulses occur at the material interfaces indicated by horizontal dashed lines, where the reflection and transmission occur according to the acoustic impedance. 2,55,56 The occurrence of multiple unipolar strain pulse echoes that traverse the TbFe 2 and Nb layers at different timings is readily seen in the UXRD data presented in Fig. 3b).…”

Studies of electron diffusion in photo-excited Ni using time-resolved X-ray diffraction

“…The laser fluence was 4 mJ/cm 2 incident on the sample with an uncertainty of about 30%, partly related to a non-Gaussian beam shape and experimental difficulties in maintaining laser/X-ray overlap during the measurement. The experimental setup has been described by Persson et al 13 and the beamline is described in more detail by Harbst et al 17 A measured time-integrated energy scan is shown in Fig. 2.…”

“…6,7 Time-resolved X-ray diffraction has been used previously to study strain/acoustic phonon generation in semiconductors and metals. 5,[8][9][10][11][12][13] The advantage of using time-resolved X-ray diffraction, rather than optical reflectivity measurements, is that the strain profile can be probed, not only at the surfaces, but also inside the material.…”

“…After calculating the lattice temperature profile T l (t,z), it was imported into Comsol V R and used to calculate the strain generation in Ni, subsequent propagation to the interface and finally the strain transmission at the interface and propagation in InSb with a temporal resolution of 0.1 ps. 13 The time dependent strain profile was sent to Sergey Stepanov's X-ray server, 19 where the time-resolved X-ray diffraction efficiency from InSb was calculated. The values used in the simulations are listed in Table I.…”