Ultrafast photoexcitation can generate internal compressive stress in Mott insulators that lead to strain waves from free surfaces. These photoinduced elastic waves can trigger phase transitions in materials. However, a comprehensive physical picture of the phase transformation dynamics that includes acoustic-scale propagation has not yet been developed. Here we demonstrate that such a strain-wave mechanism drives the ultrafast insulator-to-metal phase transition in granular thin films of the Mott material V₂O₃. Our time-resolved optical reflectivity and X-ray diffraction measurements reveal that an inverse ferroelastic shear occurs before the insulator-to-metal transition, which propagates in the wake of a compressive strain wave. These dynamics are governed by the domain size and film thickness, respectively. Our results clarify the morphological conditions for the ultrafast phase transition that is favoured in granular thin films and hindered in single crystals. The resulting physical picture sheds light on the ultrafast phase transitions in quantum materials and future devices based on Mott insulators.

超快光激发可以在莫特绝缘体中产生内部压应力，从而导致自由表面产生应变波。这些光致弹性波可以触发材料中的相变。然而，包括声学尺度传播在内的相变动力学的全面物理图像尚未开发出来。在这里，我们证明了这种应变波机制驱动莫特材料 V ₂ O ₃颗粒薄膜中的超快绝缘体到金属相变。我们的时间分辨光学反射率和 X 射线衍射测量表明，逆铁弹性剪切发生在绝缘体到金属转变之前，该转变在压缩应变波之后传播。这些动态分别由域尺寸和薄膜厚度控制。我们的结果阐明了超快相变的形态条件，该相变在颗粒薄膜中有利，但在单晶中受到阻碍。由此产生的物理图像揭示了量子材料中的超快相变以及基于莫特绝缘体的未来器件。