Backfolded acoustic phonons as ultrasonic probes in metal-oxide superlattices*

Ultrasound has been widely used as an incisive probe of internal interfaces. For thin-film structures however, it is largely ineffective because the signal is dominated by the substrate. Using confocal Raman spectro-microscopy, we show that multiple reflection of sound waves at internal interfaces of a metal-oxide superlattice generates standing waves that are insensitive to the substrate. These backfolded acoustic phonon modes are sensitive to atomic-scale thickness variations of the sublayers and thus serve as a powerful characterization tool for metal-oxide superlattices.