Second harmonic-induced ultrafast wrinkle formation in two-dimensional material

The precise control of wrinkles and strain gradients in nanofilm is of significant interest due to their profound influence on electronic band structures and spin states. Here, we employ ultrafast electron diffraction (UED) to study the picosecond-scale dynamics of laser-induced bending in 2H-MoTe₂ thin films. Owing to the sample thickness exceeding the laser penetration depth, inhomogeneous excitation leads to the emergence of both fundamental (21GHz) and second harmonic (42GHz) acoustic phonons. The experimental and simulation results demonstrate that the second harmonic directly generates a strain gradient along the c-axis, which subsequently transforms into in-plane strain through the Poisson effect. This nonuniform in-plane strain serves as the primary driving force for film bending. This study not only demonstrates ultrafast laser-based control of internal strain gradients but also provides new insights into the mechanisms of laser-induced bending mediated by coherent phonons, particularly the second harmonic.