Coherent confined LO phonons in Ge-70/Ge-74 isotope superlattices generated by ultrafast laser pulses - art. no. 161304

Generation of high-order coherent confined LO phonon modes by 20-fs ultrash ort laser pulse irradiation was observed in a new class of nonpolar semicon ductors; Ge-70/Ge-74 isotope superlattices. The phonon oscillations were Fo urier transformed and compared with a theoretical calculation based on a pl anar force-constant model and a bond polarizability approach. The compariso n 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 mod es are observable. The spectra taken with variety of polarization of pump b eam show clearly that the generation mechanism of coherent phonons in nonpo lar semiconductor like Ge is stimulated Raman scattering.