We used femtosecond time-resolved surface second-harmonic generation
to observe coherent polarization and dephasing measurement of surface electronic
states. This is the first all optical experiment to probe coherent effects
of surfaces states. Our experiments were conducted on the relaxed 1X1 reconstructed
surface of n-doped GaAs(110) prepared in an ultrahigh vacuum chamber. Two
time-delayed 20 fs, 850 nm, laser pulses were incident on the sample at
45 degrees angle of incidence with 1 degree angular separation. Second-harmonic
(SH) generation along the probe direction was detected as a function of
time-delay. Near zero time delay (pump and probe pulse overlap) the time-resolved
SH signal shows oscillations at ~3 fs period corresponding to the period
of the fundamental wavelength. This interference at optical frequencies
is due to the coherent superposition of the second harmonic driven by 2
probe photons (the usual way) and the second harmonic driven by the pump
initiated surface polarization and 1 probe photon. The latter contributes
in this case because of the small size of the laser spot. By comparison
to four-wave mixing, SH is surface sensitive and thus allows sensitivity
to surface states. The interference is explained by directly solving the
optical Bloch equations.