Coherent phonon spectroscopy of GaInP/GaAs/GaInP single quantum wells is demonstrated with time-resolved second-harmonic generation. Coherent longitudinal optical phonons are impulsively launched via transient pump-induced field screening and stimulated Raman scattering in the well and barrier regions. A phonon mode at 9.4 THz is identified as interfacial phonon localized in the GaAs/GaInP hetero-interface. The free induced dephasing of this coherent interfacial phonon is analyzed with window-gated Fourier transform. Its dephasing time is used to characterize the interface quality of semiconductor heterostructures. (C) 2004 American Institute of Physics.