Observation of local-interfacial optical phonons at buried interfaces using time-resolved second-harmonic generation

We have used an all-optical technique to observe optical phonons localized at a buried interface. The technique is based on coherent excitation of the interfacial vibrational mode by femtosecond laser irradiation and detection of the free-induction decay of the coherent oscillation by time-resolved second-harmonic generation. For native oxide-covered GaAs (100), we observe a single-optical phonon mode at $8.48\pm 0.04\mathrm{THz}$ localized to a few monolayers on the semiconductor side of the interface. The assignment is based on changes in the phonon spectrum as a function of pump-laser intensity and during in situ oxidation. The mode frequency shifts from 8.48 to 8.29 THz due to coupling with holes driven to the interface by the depletion field.