Elastically driven cooperative response of a molecular material impacted by a laser pulse

Photoinduced phase transformations(1,2) occur when a laser pulse impacts a material, thereby transforming its electronic and/or structural orders, consequently affecting the functionalities(3-7). The transient nature of photoinduced states has thus far severely limited the scope of applications. It is of paramount importance to explore whether structural feedback during the solid deformation has the capacity to amplify and stabilize photoinduced transformations. Contrary to coherent optical phonons, which have long been under scrutiny(8-10), coherently propagating cell deformations over acoustic timescales(11-14) have not been explored to a similar degree, particularly with respect to cooperative elastic interactions. Herein we demonstrate, experimentally and theoretically, a self-amplified responsiveness in a spin-crossover material(15) during its delayed volume expansion. The cooperative response at the material scale prevails above a threshold excitation, significantly extending the lifetime of photoinduced states. Such elastically driven cooperativity triggered by a light pulse offers an efficient route towards the generation and stabilization of photoinduced phases in many volume-changing materials.