<i>Picosecond laser ultrasonic measurements of elastic properties of transition metal dichalcogenides</i>

We report ultrafast optical pump-probe measurements of sound velocity and attenuation for GHz frequency longitudinal acoustic strain pulses in MoSe₂ and WSe₂. High-quality bulk single crystals of 2H-MoSe₂ and WSe₂ were synthesized by chemical vapor transport with iodine as the transport agent. Thin layers with thicknesses ranging from a few nm up to a few hundred nm were then mechanically exfoliated onto sapphire or SiO₂/Si wafers. A degenerate optical pump-probe experiment was performed with a Ti: sapphire ultrafast laser with peak wavelength varied from 760 nm to 830 nm. At these wavelengths, the optical absorption in these crystals is strong enough that a high frequency acoustic strain pulse with frequency components approaching 100 GHz is generated. This picosecond ultrasonic pulse travels back and forth in the exfoliated layer and causes a reflectivity change that is measured by time-delayed probe pulses via the strong strain dependence of the optical properties. Longitudinal sound velocities as well as an upper bound on attenuation of 100 GHz phonons are determined by comparing reflectivity data to 1D simulations.