Optical second harmonic generation can be used to probe a number of important properties of liquid crystals, including polar ordering, orientation of molecules in monolayers, and the in-plane symmetry of the molecular arrangement at various interfaces. We have used second harmonic generation to investigate the effects of liquid crystal/substrate interactions on the alignment of bulk liquid crystal.

W. Chen;M. Feller;P. Guyot-Sionnest;C. S. Mullin;H. Hsiung;元壤 沈

Proceedings of SPIE - The International Society for Optical Engineering

1989-8-15

Difference‐frequency mixing of two pump waves can in principle excite two coherent phonon waves via the parametric process. Only when the phonon excitation is small can the nonlinear susceptibility of two‐phonon coherent anti‐Stokes Raman scattering be described as proportional to the product of two Raman tensors.

元壤 沈

Journal of Raman Spectroscopy

1981-1

An experiment was designed to measure sum-frequency generation (SFG) spectra in both reflection and transmission. The result from an octadecyltrichlorosilane monolayer on a substrate reveals that while the resonant spectra in the two geometries both originate from the surface monolayer, the bulk substrate has a significant contribution to the nonresonant background in the transmitted spectra. Study of a polyethylene film also shows that the bulk contribution to the SFG spectra is only significant for the transmission geometry, exhibiting a resonant mode not observable in the reflected spectra.

Xing Wei;Seok Cheol Hong;A. I. Lvovsky;Hermann Held;元壤 沈

Journal of Physical Chemistry B

2000-4-13

We have studied the initial stages of oxidation, the temperature dependence of the surface electronic structure, and the effect of phosphorus on oxidation of the Si(111)-(7 × 7) surface using optical second-harmonic generation. We have also observed a (√3 × √3)R30° LEED pattern due to surface reconstruction induced by < 0.5% P on Si(111).

H. W.K. Tom;X. D. Zhu;元壤 沈;G. A. Somorjai

Surface Science

1986-3-1

The second-order non-linear polarizabilities of a number of organic molecules have been determined by measuring the optical second-harmonic generation from monolayers of such molecules spread on a water surface.

Th Rasing;G. Berkovic;元壤 沈;S. G. Grubb;M. W. Kim

Chemical Physics Letters

1986

The authors used the wavelength modulation technique to measure the derivative spectra dR/Rd omega of single crystals of PbS, PbSe, PbTe, and SnSe between 1. 5 and 6 eV at several temperatures from 5 to 300 degree K. The samples were cleaved and stored in the He atmosphere before measurements. As a consequence, surface contamination was avoided. They observed all the structures previously detected by normal reflectivity and electroreflectance measurements. In addition, many new structures were seen. The spectra generally become better resolved at low temperatures, but some of the structures show stronger temperature dependence than others in both the position and the strength. In particular, one component in the E//3-peak depends very strongly on temperature and becomes very pronounced through sharpening at low temperatures. The spectra were interpreted in terms of interband transitions with the available band structures for these compounds.

S. Kohn;Y. Petroff;P. Y. Yu;元壤 沈

J Nonmet

1973

Luminescence spectra of Cu2O excited at various temperatures by a tunable dye laser showed no evidence of the existence of excitonic molecules (or biexcitons) as suggested recently by Gross and Kreingol'd. A pair of previously unreported peaks with an exceptionally strong temperature dependence has been observed below 3°K.

Y. Petroff;P. Y. Yu;元壤 沈

Physical Review Letters

1972

Four-wave mixing processes with double and triple resonances are discussed in detail. Explicit expressions for the resonant nonlinear susceptibilities of various cases are derived. It is shown that in some cases, four-wave mixing can yield a spectrum with reduced inhomogeneous broadening. Possible applications of multiresonant four-wave mixing as a spectroscopic technique are considered. They include high-resolution spectroscopy, deduction of transition matrix elements, study of transitions between excited states, measurement of longitudinal relaxation times, distinction between resonant Raman scattering and resonant fluorescence, etc. Selective polarization arrangement allows us to suppress the nonresonant background, separately measure the real and imaginary parts of the resonant nonlinear susceptibility, minimize the effects of absorption and laser intensity fluctuations, and greatly enhance the signal-to-noise ratio.

J. L. Oudar;元壤 沈

Physical Review A

1980

Quantitative yet simple analytical solutions of surface temperature under pulsed laser illumination are presented for a quick estimation in optical spectroscopy studies. Dependence of steady state surface temperature as well as its temporal evolution on laser parameters, such as repetition rate and beam radius, together with medium properties is thoroughly investigated using the analytical solution, which is supported by numerical simulation. It is found that when the pulse number is larger than 100 within the heat diffusion time, the steady-state temperature rise reaches more than 85% of the temperature rise induced by CW laser heating of the same power. We provide a summary of the results to allow their use for a quick estimate of surface temperature evolution from pulse laser heating if laser parameters and medium properties are known.

Ying Xu;Ruizhe Wang;Shaojie Ma;磊 周;元壤 沈;Chuanshan Tian

Journal of Applied Physics

2018-1-14

In-situ X-ray Absorption Spectroscopy (XAS), Raman Spectroscopy, AFM and XPS have been used to investigate the effect of reactions occurring in aqueous electrolytes on the structure of a single-layer graphene produced by CVD. It was found that defects are readily and irreversibly produced by application of electrode voltages. The defects and the products were identified also by new features in the XAS spectra. Our findings show the poor stability of the CVD graphene, which could be a challenge in applications such as super-capacitors, fuel-cells, batteries and photo-catalysis.

Juan J. Velasco-Velez;Cheng Hao Chuang;Hui Ling Han;Inigo Martin-Fernandez;Camille Martinez;Way Faung Pong;元壤 沈;Feng Wang;跃钢 张;Jinghua Guo;Miquel Salmeron

Journal of the Electrochemical Society

2013