M. Nakajima, H. Harima, M. Hangyo, K. Morita, K. M. Itoh, K. Mizoguchi, and E. E. Haller, "Observation of coherent confined LO phonons in 70Ge/74Ge isotope superlattices,"
in Ultrafast Phenomena, T. Li, ed.,
Vol. 43 of OSA Trends in Optics and Photonics
(Optical Society of America, 2000), paper MF52.