THz coherent acoustic phonons on a scale of sub-picoseconds in temporal-periodicities and nanometers in wavelengths are promising in various fields such as nanometrology/nanoimaging, THz devices and heat management. By merging advanced ultrafast laser spectroscopy and sophisticated nanotechnologies, the excitation and detection of bulk coherent acoustic phonons in the THz regime have been accomplished. Nevertheless, coherent surface acoustic waves (SAWs) realized by metallic gratings deposited on a substrate, are still below 100 GHz. In this report, we take advantage of the cleaved superlattice (SL) surface with immediately reachable nanometer periodicity and atomic-level interface quality to monitor SAWs by femtosecond lasers. Rayleigh SAWs above 100 GHz and bulk surface skimming acoustic waves up to 1THz, with deeply sub-optical-wavelength periodicities, are observed on cleaved Al0.3Ga0.7As/GaAs SLs and cleaved In0.2Ga0.8N/GaN SLs, respectively. The observations open a path towards THz opto-acoustic/acousto-optic transducers.
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