Generation and Detection of Coherent GHz Surface Phonons in Superlattices

The development of transducers to excite/detect coherent surface acoustic waves (SAWs) with picosecond periods and nanometer-level wavelengths and localization depths would be very useful for nanometrology, nanoimaging, sensing and manipulations in fundamental and applied research. Earlier, the control of SAWs and Lamb waves up to the record frequencies of about 100 and 200 GHz, respectively, was achieved by applying fs pump-probe laser pulses to metallic gratings deposited on a substrate. To reach 1 THz phonons, the grating period should be shorter than 3 nm with a velocity of 3 km/s, which is hardly achievable with conventional techniques. We suggest to engineer unconventional transducers based on cleaved nanostructured bulk materials to address this issue [1]. Currently, bulk superlattices (SLs) with a period of several atomic layers can be grown by epitaxy. They can be cleaved along the direction normal to their layers, producing a periodically nanopatterned surface. We report here first experimental monitoring by lasers of coherent phonons up to 70 GHz on the cleaved cross sections of Ga_1-x Al_xAs/Ga_1-yAl_yAs SLs. [1] V.E. Gusev, In: D. Bićanić (eds) Photoacoustic and Photothermal Phenomena III. Springer Series in Optical Sciences, 69, 323 (Springer, Berlin, Heidelberg, 1992).