Optically-excited acoustic waves in Si nanowires probed by time-resolved HOLZ lines

Exploring advanced techniques capable of probing nanometric acoustic waves in nanostructures is critically important for the development of miniaturized acoustic devices. In this study, we probe the optically-excited acoustic waves in a single silicon nanowire (NW) using the time-resolved (tr-) high-order Laue-zone (HOLZ) lines under convergent-beam electron diffraction (CBED) conditions in an ultrafast transmission electron microscope (UTEM). We devise an experimental scheme to obtain tr-HOLZ lines under off-zone-axis CBED conditions. We also propose a geometric description of HOLZ line formation and use this alternative description to quantitatively evaluate the dynamics of optically-excited silicon NW. Using part of the deformation gradient tensor, our simulations of the dynamics of Si NW reproduce the experimental results. We further discuss the feasibility of a full retrieval of the deformation gradient tensor by using a set of HOLZ lines from three zone axes. Our findings illustrate a strategy for the quantitative access to dynamical acoustic waves optically excited in micro- and nano-structures using UTEM.