Enhancement of Brillouin scattering by high-frequency excitation of ultrasound from a thin-film transducer

Brillouin scattering techniques enable non-contact velocity measurements in minute areas. However, the very weak intensity of Brillouin scattering results in long measurement times and requires high measurement stability. One approach to overcome this issue is to enhance Brillouin scattering by generating coherent phonons using a thin-film transducer (ScAlN thin film on a quartz substrate). In our study, a glass plate sample (1 mm thick) was placed on the transducer with a thin coupling medium (water). For Brillouin scattering measurements, a solid-state laser (532 nm) and Tandem Fabry-Perot interferometer (Table stable ltd.) were used. The Reflection-Induced Theta-Angle scattering geometry was used to measure in-plane longitudinal waves. A 0.95–1.05 GHz chirp signal from a network analyzer (E5071C, Agilent Technologies) was applied to the transducer to generate longitudinal waves. By varying the input voltage to the transducer, we observed an increase in only one side of the Brillouin peaks (Stokes or Anti-Stokes), indicating phonon propagation in a single direction. The peak intensity was enhanced by a factor of four due to the coherent phonons. This enhancement system is very simple, and samples can be measured easily and nondestructively.