Generation and detection of coherent picosecond surface acoustic waves in superlattices

The development of transducers to excite and detect coherent surface acoustic waves (SAWs) with picosecond periods and deeply-sub-optical, nanometer-level wavelengths and localization depths would be very useful for nanometrology, nanoimaging, sensing and manipulations in fundamental and applied research. The control of SAWs and Lamb waves up to the record frequencies of 90 and 197 GHz, respectively [1,2], was achieved by applying femtosecond pump-probe laser pulses to the metallic gratings deposited on the substrate. The spectrum of the pump-pulse intensity envelope contains the required frequencies, while the periodicity of the grating induces required wave numbers in the spatial distribution of the laser-induced mechanical stresses. The grating period should be shorter than 3 nm for 1 THz SAWs with velocity of 3 km/s. Yet the minimum period of the structures currently patterned in the laboratories is just approaching 10 nm. The key limitation in SAW generation at frequencies approaching 1 THz is therefore due to limitations of surface nanopatterning. Here, we suggest an engineering for SAWs/Lamb waves monitoring of unconventional transducers based on cleaved/sliced nanostructured bulk materials [3]. Currently, bulk superlattices (SLs) with a period of several atomic layers can be grown by epitaxy. They can be cleaved/sliced along the direction normal to their layers producing a periodically nanopatterned surface. We report first experimental monitoring of the coherent SAWs and surface skimming bulk acoustic waves up to 70 GHz on the cleaved cross sections of Ga1-x AlxAs/Ga1-yAlyAs SLs. We also discuss the optical detection mechanism of deeply-sub-optical wavelength acoustic excitations in the SLs. [1] M. Shubert, et al., Appl. Phys. Lett. 101, 013108 (2012). [2] M. Grossmann, et al., Appl. Phys. Lett. 106, 171904 (2015). [3] V. E. Gusev, In: D. Bicanic (eds) Photoacoustic and Photothermal Phenomena III. Springer Series in Optical Sciences, 69, 323 (Springer, Berlin, Heidelberg, 1992).