Phononic measurement of transparent materials.
In (a), a light pulse (pump) is used to thermoelastically generate a coherent phonon field in the metallic film. The phonon field (shown at two positions in time, t1 and t2, by thin horizontal bars) is probed by a second light beam (probe). The interference of both direct and scattered probe beams induces an oscillation in the detected probe light intensity. The frequency of this oscillation fB is a function of the speed of sound. As the phonon field travels from one material with speed of sound ν1 to another material with speed of sound ν2, the detected frequency changes accordingly as shown in (b). As the phonon wavelength is shorter than the optical, it is possible to section the optical volume by post-processing without the need of further acquisition, mechanical positioning or change of focus.