Generation and detection of 50 GHz surface acoustic waves by extreme ultraviolet pulses

Abstract

We use femtosecond extreme ultraviolet pulses derived from a free electron laser to excite and probe surface acoustic waves (SAWs) on the (001) surface of single crystal SrTiO3. SAWs are generated by a pair of 39.9 nm pulses crossed at the sample with the crossing angle defining the SAW wavelength at 84 nm. Detection of SAWs is performed via diffraction of a time-delayed 13.3 nm probe pulse by SAW-induced surface ripples. Despite the low reflectivity of the sample in the extreme ultraviolet range, the reflection mode detection is found to be efficient because of an increase in the diffraction efficiency for shorter wavelengths. We describe a methodology for extracting the SAW attenuation in the presence of a thermal grating, which is based on measuring the decay of oscillations at twice the SAW frequency. The proposed approach can be used to study ultrahigh frequency SAWs in a broad range of materials and will bridge the wave vector gap in surface phonon spectroscopy between Brillouin scattering and He atom scattering.

Phonon transport in the gigahertz to terahertz range: Confinement, topology, and second sound

“…Frequency-domain methods include Raman thermometry 15,16 , Brillouin light scattering (BLS) 17 and frequency-domain thermoreflectance (FD-TR) 18,19 . Time-domain methods include transient grating techniques [20][21][22] , time-resolved thermoreflectance (TD-TR) 23,24 , time-domain BLS 25 and the method of asynchronous optical sampling (ASOPS) 26,27 . In this work, we emphasize experimental works and particularly the techniques of BLS, FD-TR and TD-TR.…”

Extreme ultraviolet transient gratings: A tool for nanoscale photoacoustics

“…5 (a) illustrates an example of this trade-off for different samples, assuming pm (in the same range as found in Ref. [34] for the case of SrTiO 3 ) and typical values of incident and diffraction angles ( = = 15°), while the values of R are taken from Ref. [52] .…”

“…The EUV TG approach has been pioneered at the FERMI FEL (Trieste, Italy) with the dedicated endstation TIMER [32] . First results beyond the proof of principle [33] reveal that EUV TG is capable of incisively and selectively studying bulk and surface phonons [4] , [34] , thermal transport kinetics [14] , [35] and magnetic dynamics [36] , [37] . The recent development of the technique has revealed specific peculiarities with respect to the well-established optical TG spectroscopy that need to be considered both when predicting as well as when interpreting experimental EUV TG results.…”

Probing electron and hole colocalization by resonant four-wave mixing spectroscopy in the extreme ultraviolet

“…From an experimental point of view, we note that the third-order nonlinear process studied in this work, i.e., SFG and DFG, allows for easy spectroscopic discrimination of the nonlinear signal from the incident XUV or x-ray radiation, which is particularly important when mixing harder x-rays with laser pulses in the optical spectral range due to the challenge posed by an increasingly small angular separation. Furthermore, we note that the observation of a nonlinear process in reflection from a solid sample–vacuum interface is convenient in particular in the XUV spectral range ( 27 , 28 ), because, due to strong absorption, only very thin samples can typically be used to detect the generated radiation propagating through the nonlinear medium, e.g., a sample of 50-nm thickness was used in ( 13 ). We expect both these aspects as demonstrated here to be of relevance for the application of nonlinear x-ray spectroscopy in materials science, where tailoring the samples under study to experiments is only viable to a limited extent.…”