  Terahertz (THz) time-domain emission spectroscopy was performed on layered
2H-MoSe2 and 2H-WSe2. The THz emission shows an initial cycle attributed to
surge currents and is followed by oscillations attributed to coherent
interlayer phonon modes. To obtain the frequencies of the interlayer
vibrations, analysis of the THz emission waveforms were performed, separating
the two contributions to the total waveform. Results of the fitting show
several vibrational modes in the range of 5.87 to 32.75 cm-1 for the samples,
attributed to infrared-active interlayer shear and breathing modes. This study
demonstrates that THz emission spectroscopy provides a means of observing these
low frequency vibrational modes in layered materials.
