Picosecond laser ultrasonic measurements of interlayer elastic properties of 2H-MoSe2 and 2H-WSe2

We have used the ultrafast pump-probe technique known as picosecond ultrasonics to generate and detect coherent acoustic phonon pulses with frequencies reaching 40 GHz in exfoliated crystals of MoSe2 and WSe2 on Si and sapphire substrates. We report picosecond time-resolved reflectivity data from samples ranging from 180 nm to 920 nm in thickness and compare our results to a 1D simulation of strain-induced changes in the optical reflectivity. We find the longitudinal sound velocity along the c-axis (the interlayer direction) to be 2800 m/s ± 40 m/s for MoSe2 and 2510 m/s ± 60 m/s for WSe2. We also report the measured lifetime of longitudinal acoustic phonons approaching 40 GHz to be 0.85 ± 0.2 ns and compare this value with predictions of relaxation damping and 3-phonon models, as well as discuss its relationship to the predicted thermal conductivity of MoSe2.