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

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.

Automated summary

In this study, femtosecond extreme ultraviolet pulses were used to excite and probe surface acoustic waves on the (001) surface of single crystal SrTiO3. Detection of SAWs was achieved through diffraction of a time-delayed probe pulse, despite the low reflectivity of the sample in the extreme ultraviolet range. A methodology for extracting SAW attenuation in the presence of a thermal grating was described, which can be used for studying ultrahigh frequency SAWs in various materials and bridging the wave vector gap in surface phonon spectroscopy.