Generation of high-order coherent confined LO phonon modes by 20-fs ultrashort laser pulse irradiation was observed in a new class of nonpolar semiconductors; ${}^{70}\mathrm{Ge}{/}^{74}\mathrm{Ge}$ isotope superlattices. The phonon oscillations were Fourier transformed and compared with a theoretical calculation based on a planar force-constant model and a bond polarizability approach. The comparison between the calculated and Fourier transformed spectra shows clearly that the amplitudes of coherent phonons are determined solely by the degree of the atomic displacement and that only the Raman active odd-number-order modes are observable. The spectra taken with variety of polarization of pump beam show clearly that the generation mechanism of coherent phonons in nonpolar semiconductor like Ge is stimulated Raman scattering.

• Received 28 July 2000

DOI:https://doi.org/10.1103/PhysRevB.63.161304