Laser-induced coherent longitudinal acoustics phonons, in thin films, observed by ultrafast optical reflectivity and ultrafast X-ray diffraction*

Achieving ultrafast modulations of material properties is an important research topic. Here, we present our experimental data on the propagation of laser-induced coherent longitudinal acoustic phonons (CLAPs) at thin-film interfaces and heterojunctions using ultrafast optical reflectivity and ultrafast x-ray diffraction measurements. We observe that CLAPs can efficiently propagate from a LaMnO_3 thin-film to its SrTiO_3 substrate owing to the matching of their acoustic impedance, and the oscillation period increases from 54 GHz to 105 GHz. In contrast, in ultrafast X-ray diffraction experiments, we discover that CLAPs are partially confined within an Au (111) thin film due to the mismatch of acoustic impedance with the substrates, leading to an oscillation period of 122 ps. However, when examining La_0.7Ca_0.175Sr_0.125MnO_3/Ba_0.5Sr_0.5TiO_3 bilayers, no oscillations are observed due to the favorable impedance matching between the layers. Our findings demonstrate that acoustic impedance can serve as an effective means to control coherent phonons in nanometer-thin films and may also play a crucial role in phonon engineering at interfaces or heterostructures.