No abstract

Second-harmonic generation ͑SHG͒ was used to investigate chemically modified surfaces of Ge͑111͒. Chemical modification was achieved by wet-chemical covalent binding of decyl and sulfur directly to the Ge interface after oxide stripping. Chemical modification of the interface substantially changes the second harmonic response. The decyl and sulfur terminations are stable in ambient during several weeks, as judged by SHG and XPS measurements. The SHG rotational anisotropy patterns were analyzed to estimate the relative values of the nonlinear susceptibilities describing the surface and bulk response. The choice of fundamental/SHG polarization combinations for accessing various nonlinear coefficients is presented. The factors affecting the relative values of the surface-to-bulk contributions to SHG and their changes upon chemical modification of the surface are discussed. In particular, it was found that the higher the electronegativity of chemically attached species, the higher the contribution of the surface-originating nonlinear terms to the overall response. Also, it was found that the relative contribution of surface versus bulk to SHG is different for different polarization combinations: the surface contribution to the p-in/p-out response is the greatest.

The oxidation of H terminated silicon surfaces is a significant and controversial problem in silicon device fabrication. Second-harmonic generation rotational anisotropy (SHG–RA) provides a convenient means to monitor the chemical state of the Si surfaces, and to follow the conversion of H terminated surface to SiO2 by oxidation as a function of time in ambient. The change in SHG–RA of Si(111)–H was shown to correlate well with the ellipsometric thickness. SHG is sensitive to the initial stage of oxidation (induction period) as well as to the logarithmic oxide growth. SHG is sensitive to the electronic properties of the surface, therefore it is a sensitive probe of the quality of H terminated Si(111) surface. Under ambient conditions, (20% relative humidity, 23 °C) the initial oxidation rate is at most 2×10−6 ML/s.

An interface specific investigation, by time-resolved second-harmonic generation, shows that photoexcited carrier dynamics at Si͑111͒ interfaces depend strongly on surface termination. Oxideand H-terminated surfaces show distinct transient behavior, with a surface recombination velocity Ͻ10 3 cm/s. Incompletely H-terminated Si͑111͒ shows faster dynamics, correlating with less interface passivation. A simple model reveals that the second-order nonlinear optical susceptibility of photoexcited carriers is two orders of magnitude greater than that of the valence band electrons.

A tunable, narrow bandwidth, high peak power picosecond infrared ͑IR͒ laser system is described. The pump source is a picosecond Ti:sapphire regenerative amplifier seeded by a picosecond Ti:sapphire oscillator. The pump bandwidth and pulse duration are tunable producing 4-5 ps, 5-4 cm Ϫ1 pulses at 1 kHz. IR pulses are produced by optical parametric generation ͑OPG͒ followed by optical parametric amplification ͑OPA͒. Tuning is possible over the entire 1050-3300 nm region of the IR, with energies in excess of 15 J over most of the range. The temporal and spectral characteristics of the IR pulses are reviewed with a particular focus on the sources of bandwidth broadening in the OPG/OPA. Bandwidth optimization of the IR output is discussed. A spectral filtering scheme results in less than 15 cm Ϫ1 IR bandwidth, suitable for nonlinear optical spectroscopic applications.