We studied GHz hypersound in heterostructures of ${\mathrm{SrRuO}}_3$ (SRO) and ${\mathrm{SrTiO}}_3$ (STO) by using femtosecond pulses with fluence as low as several tens of $\mu \mathrm{J}/\mathrm{c}{\mathrm{m}}^2$. We found that the efficiency of generating hypersound in SRO, through thermal expansion, is linear up to optical fluence of a few $\mathrm{mJ}/\mathrm{c}{\mathrm{m}}^2$. We further investigated the propagation of acoustic pulses in the STO substrate. The acoustic properties of STO dramatically changed around the temperature of structural transition (105 K) because the generated hypersound was coupled to the softening mode. Nevertheless, our experimental results revealed that the quadratic frequency dependence of acoustic attenuation in STO, predicted by the mode-coupling theory, breaks down at tens of GHz. Finally, the refractive index of STO was experimentally obtained between 365 and 400 nm, for which there was disagreement in previous results.

• Received 2 November 2017
• Revised 15 August 2018

DOI:https://doi.org/10.1103/PhysRevB.98.064308