Nanolayer-Confined Strain Waves in Femtosecond-Optical-Vortex-Excited Multiferroic BiFeO3.

Strain waves provide a route to reshape nanoscale order in functional materials, where lattice deformation couples to charge and spin degrees of freedom. Real-space tracking of these transient strain fields is critical for connecting acoustic excitation to functionality, yet progress has been limited because buried crystalline layers require simultaneous spatial, temporal, and crystallographic selectivity for high-resolution imaging beyond the surface. Here, we reveal optical-vortex-excited strain dynamics in a 100 nm thick BiFeO3 layer embedded in an oxide heterostructure using time-resolved dark-field X-ray microscopy. An infrared optical vortex launches strain waves, and (001)PC Bragg-reflection images track the [001]PC-projected lattice deformation, revealing acoustic modes at 15.7, 21.0, and 35.5 GHz and a longitudinal sound velocity of 4.10 ± 0.18 km s-1. Two-temperature finite-element simulations attribute the transient angular shifts to overlapping tensile and compressive strain waves. This work establishes a route to visualize confined picosecond strain fields in buried functional nanolayers.