Generation and control of ultrashort-wavelength two-dimensional surface acoustic waves at nanoscale interfaces

In this work, we generate and probe the shortest wavelength surface acoustic waves to date, at 45 nm, by diffracting coherent extreme ultraviolet beams from a suboptical phononic crystal. The short acoustic wavelengths correspond to penetration depths of approximately 10 nm. We also measure the acoustic dispersion in two-dimensional nanostructured phononic crystals down to this wavelength for the first time, showing that it is strongly influenced by the ultrashort acoustic penetration depth, and that advanced finite-element analysis is required to model the dispersion. Finally, we use pulse sequences to control surface acoustic wave generation in one-dimensional nanostructured gratings, to preferentially enhance higher-order surface waves, while suppressing lower frequency waves. This allows us to reduce the generated surface acoustic wavelength by a factor of two for a defined nanostructure period.