Observation of coherent surface optical phonon oscillations by time-resolved surface second-harmonic generation

We demonstrate a new time-domain surface phonon spectroscopy. We excite coherent surface optical phonons with an ultrashort laser pulse and probe the free-induction decay with time-resolved surface second-harmonic generation. For both clean GaAs (110) and (100) surfaces, the signals are remarkably large due to the highly localized electron-phonon interaction at the surface and the intrinsic-surface sensitivity of second-harmonic generation. This work suggests that coherent oscillations of surface optical phonons may be used as probes of electron-phonon interactions at surfaces and to control or drive nonthermal surface chemical reactions.