<i>Ultrafast optical measurements of elastic properties of 2H-MoSe₂</i>

We report ultrafast optical studies of the velocity and lifetime of 30 - 40 GHz longitudinal acoustic phonons in 2H-MoSe₂. High-quality bulk single crystals of 2H-MoSe₂ were synthesized by chemical vapor transport with iodine as the transport agent. Thin layers ranging from a few nm up to a few 100 nm were then mechanically exfoliated onto sapphire or Si wafers. A degenerate pump-probe experiment was performed with a Ti:sapphire laser with central wavelength varied from 760 nm to 830 nm. In the near-IR the optical absorption is strong enough that an acoustic strain pulse with frequency components greater than 40 GHz is generated. This acoustic pulse travels back and forth in the MoSe₂ and causes a change in reflectivity that is measured by time-delayed probe pulses via the strong dependence of the optical properties of the crystal on strain. The measured sound velocity (2800 ±40 m/s) is in good agreement with previously published values, while the measured phonon lifetime (0.85 ± 0.2 ns) is a factor of 2 lower than a recent measurement by a related technique on a free standing membrane. We discuss implications for thermal conductivity and demonstrate GHz acoustic frequency comb behavior in the data.