Picosecond ultrasonics is an efficient technique to generate and detect acoustic phonons in the sub-THz range.1 The development of transducers based on nanostructures, such as superlattices, quantum well or quantum dots, has brought the generation of acoustic phonons in the THz range.2 With such frequency, the possibility to realize high in-depth resolution acoustic imagery is reachable. However, the achievable lateral size is a great limitation to the imaging capabilities of picosecond ultrasonics. Since the acoustic phonons are generated by laser, the lateral size is limited by the area of the focused laser beam.
To overcome this limitation, different approaches have been investigated. For example, a better resolution can be obtained by using a near-field scanning optical microscope.3 It is also possible to take advantage of the transducer properties.4 Another possibility is to use nanostructure. In that case, the lateral size of the generated acoustic phonons is given by the size of the nanostructure. Nanowires are good candidates, however, up to now only confined acoustic modes have been observed. Therefore a transducer able to generate acoustic phonons propagating in the nanowire has to be designed. In this study, we propose to include a superlattice inside nanowires to realize the transduction of acoustic phonons that propagates in this structure. The modifications induced by the confinement on the generation by the superlattice are investigated. Then, by taking advantages of the excellent detection capabilities of superlattices, we studied the dispersion of acoustic phonons after their propagation in the nanowire. The observation of these guided acoustic phonons shows the potential of such system for low lateral size acoustic transducer.