27 Avril – Thesis defense - Louise Le Ridant

10 h30 En visioconférence

Design of a multilayer opto-acoustic transducer for picosecond ultrasonics.

Picosecond ultrasonics is a contactless technique that offers the possibility to optically generate and detect acoustic waves with an unequalled frequency bandwidth (above GHz). With femtosecond lasers pulses, it measures the reflectivity change caused by acoustic waves which propagate in the whole structure and which are generated by the photo-thermo-elastic effect. Its sub-micrometric spatial resolution and its non-invasive nature allow the mechanical characterization of micrometric structures such as thin layers or biological cells. In order to improve the sensitivity of the measurement to acoustic disturbances, we present in this work the design of an innovative transducer. To maximize the signal to noise ratio, we use the sensitivity of an optical cavity to changes of its thickness. A predictive code based on the transfer matrix formalism has been developed to study the physical response of a multilayer transducer composed of transparent and absorbing layers. Thanks to both the simulations of optical and acoustical fields, we were able to design and build a three-layer transducer. Both reflectivity measured in reflexion and transmission were compared to simulation results. Then, the sensitivity of a new transducer to acoustic disturbances was studied according to the wavelength of the probe beam. Finally, we characterize the amplitude gain obtained by the use of the new transducer in relation to usual transducer.

Event localization