Coherent longitudinal acoustic phonon approaching THz frequency in multilayer molybdenum disulphide

Coherent longitudinal acoustic phonon is generated and detected in multilayer Molybdenum Disulphide (MoS₂) with number of layers ranging from 10 to over 1300 by femtosecond laser pulse. For thin MoS2, the excited phonon frequency exhibits a standing wave nature and shows linear dependence on the sample thickness. The frequency varies from 40 GHz to 0.2 THz (10 layers), which promises possible application in THz frequency mechanical resonators. This linear thickness dependence gradually disappears in thicker samples above about 150 layers, and the oscillation period shows linear dependence on the probe wavelength. From both the oscillation period of the coherent phonon and the delay time of acoustic echo, we can deduce a consistent sound velocity of 7.1110³ m/s in MoS₂. The generation mechanisms of the coherent acoustic phonon are also discussed through pump power dependent measurement.