Picosecond ultrasonics : an original tool for physical characterization of Bragg reflectors in bulk acoustic wave resonators

Here, we present the application of the picosecond ultrasonic technique to the design and the characterization of Bragg reflectors used in the insulation of bulk acoustic wave resonators. We first show that picosecond ultrasonics is helpful for the design of Bragg reflectors through precise sound velocity measurements in thin films whose elastic properties may be much different. Secondly we apply the same technique to the complete reflector and show that it permits to check the matching of the thicknesses with λ/4 and provides a direct measurement of the reflector's central frequency.