The nonlinear optical susceptibilities, |χ14(2)(2ω)| for second-harmonic generation of GaAs, InAs, and InSb in the range 0.05≤ω≤3.0 eV are calculated from the band structure obtained by the empirical pseudopotential method. The spin-orbit interaction has been taken into account, and the transitions from the top four valence bands to the bottom four conduction bands are included in the calculation. The calculated |χ14(2)(2ω)| with the k→-dependent dipole matrix elements give much better agreement with the available experimental data than the results of constant matrix elements, which, in turn, show more structure than the ones calculated by Bell. In the case of InSb, there are two structures at 1.6 and 1.8 eV due to double resonances. A simplified model to predict structure in |χ14(2)(2ω)| from the linear spectrum is discussed.

C. Y. Fong;元壤 沈

Physical Review B

1975

Optical second harmonic generation (SHG) was used to study how mechanical rubbing modifies the surface structure of a polyimide (poly[4,4′-oxydiphenilenepyromellitimide], PMDA-ODA) film and how the rubbed polyimide surface aligns a liquid crystal monolayer (4′-;z-octyl-4-cyanobiphenyl, 8CB) adsorbed on it. It was found that rubbing would align the polymer chains and make the polymer surface more polar and hence more SHG active. The rubbed polymer surface would then align the adsorbed 8CB monolayer via molecule-molecule interaction in an orientational epitaxy manner. Approximate orientational distributions for molecules at the rubbed polymer surface and in the aligned 8CB monolayer were deduced from the measurements.

Masahito Oh-e;Seok Cheoi Hong;元壤 沈

Journal of Physical Chemistry B

2000-8-10

Photoluminescence was used to study multiphoton excitations in porous Si. With picosecond exciting pulses in the mid-infrared, even a nine-photon excitation process could be readily observed. Excitations are likely of the multi-resonant, multi-step type, with the surface states involved as intermediate resonant levels. Absorption by vibrational transitions of surface species could lead to resonant features in the excitation spectrum.

D. Kim;I. H. Libon;C. Voelkmann;V. Petrova-Koch;元壤 沈

Materials Science and Engineering B: Solid-State Materials for Advanced Technology

1997-8-1

A theory for the highly forbidden resonant Second-Harmonic Generation (SHG) process in atomic vapor is developed using a dc-field-induced SHG model. Time developments of the laser-ionization-initiated dc electric field and the coherent resonant excitation of atoms induced by nanosecond, picosecond, or femtosecond laser pulses are calculated. The dc-field-induced SHG is then predicted for different cases. Predictions including input-intensity dependence, vapor-density dependence, and spatial profile of SHG agree well with available experimental results.

D. Kim;C. S. Mullin;元壤 沈

Applied Physics B: Lasers and Optics

1995

Surface vibrational spectroscopy is one of the best means for characterizing molecular absorbates. However, most of the techniques developed so far suffer from poor sensitivity, low spectral and temporal resolution, and applications are limited to vacuum-solid interfaces. Second harmonic generation (SHG), which has been proved to be a simple but versatile surface probe, is highly sensitive and surface specific and capable of achieving high temporal, spatial, and spectral resolution. It can be applied to any interface accessible by light. Its lack of molecular selectivity can be remedied by extending the technique by means of IR-visible sum frequency generation (SFG). Surface vibrational spectroscopy with submonolayer sensitivity is then possible using SFG with the help of a tunable IR laser. Here, an SFG measurement of the C-H stretch vibration of monolayers of molecules at air-solid and air-liquid interfaces is reported. A picosecond frequency-doubled Nd:YAG laser and a picosecond optical parametric amplifier were used as the pump sources.

J. H. Hunt;P. Guyot-Sionnest;元壤 沈

1987

We report the observation of UV photoluminescence (PL) from wurtzite GaN by multiphoton excitation. The dependence of the PL intensity on excitation intensity as well as PL excitation measurements with sub-bandgap photon energies indicate the existence of deep defect states centered at about 1.0 eV above the top of the valence band. This result was confirmed by a sum-frequency excitation spectrum. We correlate these measurements with the omnipresent yellow luminescence in GaN. Our two-photon PL excitation spectrum yields a two-photon absorption coefficient that agrees very well with theoretical predictions.

I. H. Libon;C. Voelkmann;D. Kim;V. Petrova-Koch;元壤 沈

Materials Research Society Symposium - Proceedings

1997

Surface-specific IR+visible sum frequency generation (SFG) spectroscopy was used to obtain chemical composition of two polymer surfaces. The SFG surface vibrational spectrum of pure low density polyethylene and that of a commercial sample of the same kind of polymer, which contains additives, are markedly different. This correlates well with the very different surface mechanical properties, i.e., stiffness (indicative of the elastic modulus) and friction, which were measured by atomic force microscopy (AFM) on the same polymer surfaces. The surface of CLDPE is dominated by methoxy (-OCH₃) contained additives, segregated from the bulk, which explains a lower stiffness, adhesion and friction of the surface, as measured by AFM.

D. H. Gracias;D. Zhang;元壤 沈;G. A. Somorjai

Tribology Letters

1998

The steady-state response of a nonlinear medium to several simultaneously applied monochromatic electromagnetic fields is described by the density matrix method. A Fourier series expansion in terms of ascending powers of the amplitudes of the applied fields is especially useful to describe the parametric response in spectral regions, where the absorption is small. As the resonances of the material system are approached, the general formalism exhibits the mixture and interference between parametric processes and single- and multiple-photon absorption and emission processes. previously discussed examples of the two-level and three-level system are generalized. The reaction of the general nonlinear medium on the electromagnetic fields is incorporated.

N. Bloembergen;元壤 沈

Physical Review

1964

David H. Gracias;Zhan Chen;元壤 沈;Gabor A. Somorjai

Accounts of Chemical Research

1999-11

We report the first observation of a vibrational spectrum of a monolayer of molecular adsorbates by infrared-visible sum frequency generation. The results pave the way for future dynamic studies of surface phonons and molecular vibrations.

X. D. Zhu;Hajo Suhr;元壤 沈

Physical Review B

1987