Multilayer transducer for highly efficient initiation of time-resolved Brillouin scattering

Structures made of a metallic film deposited on a substrate are conventionally used as opto-acoustic transducers for picosecond ultrasonic experiments where detection in the time domain of the Brillouin scattering in a transparent sample is sought. In this paper, we substitute the metallic film for a periodic stack of nanometric layers made of gold and lithium fluoride to increase the amplitude, at the Brillouin frequency shift, of the strain generated by the photo-thermal effect. A model is used to analyze the generated strain amplification with the volume fraction and with the total thickness of this structure and to evaluate the gain in terms of sample dynamic reflectivity changes. Amplification by a factor of 20 is measured when using the composite structure with respect to signals detected with a transducer made of a single gold layer.