Photoinduced Structural Dynamics of 2H-MoTe₂ Under Extremely High-Density Excitation Conditions

Through interband photoexcitation, a representative transition metal dichalcogenide (TMD) material, MoTe₂, can undergo various phenomena such as photothermal conversion, phase transition, nonlinear optical effects, and laser ablation depending on the excitation level. However, a comprehensive study of the photoinduced structural dynamics of MoTe₂ has yet to be performed because some of these phenomena interfere in a complex manner. In the present study, the photoinduced structural dynamics of 2H-MoTe₂ was investigated under various excitation levels at a wavelength of 400 nm using ultrafast time-resolved electron diffraction and transient reflection measurements. Photoexcitation induced coherent phonons for 1-2 ps, which subsequently decayed into isotropic thermal vibrations at ∼10 ps. The amplitudes of the generated coherent phonon and thermal vibrations were found to linearly increase as the incident fluence approached 3-4 mJ/cm²; however, the amplitudes remained nearly constant when the incident fluence ranged from 4-14 mJ/cm² due to saturable absorption. Multiphoton absorption processes might be dominant above a fluence of 15 mJ/cm². Photoexcitation at high fluence (20-30 mJ/cm²) permanently damaged the sample through laser ablation and tellurium segregation. The insights in this study are critical for the further applicability and fundamental optical properties of photodevices based on TMD materials.