Coherent surface phonon at aGaAs(100)−c(8×2)surface

Coherent surface phonon at a $\mathrm{GaAs}\mathrm{}\left(100\right)-c(8\mathrm{}\times 2)-\mathrm{Ga}$ reconstructed surface has been investigated by time-resolved second-harmonic generation (TRSHG). The phonon mode is impulsively excited by an ultrashort laser pulse and subsequent coherent nuclear motion is monitored through the intensity modulation of the second harmonics of a probe pulse. Oscillatory traces are clearly observed in TRSHG signals and their Fourier transformation show two peaks at 8.2–8.6 and 8.9 THz. Fitting these traces with two oscillatory components shows that the oscillatory signals are contributed by the bulk LO phonon at 8.8 THz and the surface phonon at 6.0–8.6 THz. The relative amplitude of the surface phonon modes is sensitive to sputtering and annealing of the surface. Clear dips appear at 8.7 THz in the Fourier spectra, which is caused by the initial phase difference between the surface phonon and the bulk phonon modes. The frequency of the surface component shows red shifts as the pumping power increases. The shifts are indicative of a marked electron phonon interaction or anharmonicity of the surface phonon modes.