Control of interlayer interactions in van der Waals materials using light

In quasi 2D materials, coherent lattice distortions can be induced by a femtosecond laser pulse to guide a material into thermally inaccessible states. Here we show that interlayer interactions play a significant role in stabilizing these light-induced metastable states in 1T-TaSe₂, a material that supports an in-plane charge density wave (CDW) breathing mode. By combining multi-pulse excitation with time- and angle- resolved photoemission spectroscopy and ultrafast reflectivity, we made a surprising finding: although the band structure of the metastable CDW states were dramatically different from the ground state, the in-plane breathing mode frequency did not change. Density functional theory calculations provided further evidence of the importance of interlayer interactions and out‑of‑plane configuration, in ultrafast electronic dynamics and unique metastable atomic structures in this van der Waals material.