Annual Meeting of the American Physical Society, March 1998, Los Angeles, CA

Session W9 - Femtosecond Surface Dynamics Theory and Experiments: II.

MIXED session, Thursday afternoon, March 19
402B, Los Angeles Convention Center

[W9.06] Measurement of Surface Electronic Dephasing Time using Surface Second Harmonic Generation

Hidong Kwak, Keng C. Chou, Harry W. K. Tom (Dept. of Physics,University of California, Riverside)

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.