Coherent phonon and electron spectroscopy on surfaces using time-resolved second-harmonic generation
We demonstrate new time-domain surface phonon and surface electronic spectroscopies. We excite coherent surface optical phonons with an ultrashort laser pulse and probe the free-induction decay with time-resolved surface second-harmonic generation (TRSHG). For both clean GaAs(110) and (100) surfaces, the signals are remarkably large and we obtain an energy resolution of 1%, which is comparable to or better than that in HREELS and HAS. The phonon modes must be Raman active to be driven and hyper-Raman active to be detected through TRSHG. We obtain time-domain coherent electron spectroscopy through analysis of the pump-probe cross-correlation SH and a novel superposition of the cross-correlation and probe SH fields proportional to Epump(ω)Eprobe*(ω)|Eprobe(ω)|2. This is much easier to detect than transient grating, photon echo, or four-wave mixing schemes that use higher-order nonlinearities. We have applied this technique to measure the energy gap and dephasing time of the dangling bond interband transition on the GaAs(110)-relaxed (1×1) surface. Surface-carrier/surface-phonon interaction plays an important and perhaps dominant role in surface carrier dephasing consistent with the larger electron-phonon coupling on the surface compared to the bulk.