Optical second-harmonic generation has been used to study the effect of coadsorbed small volatile molecules on the orientational order of a liquid crystal monolayer at the interface between a mica surface and a liquid crystal film. As the chemical potential of the volatile molecules was increased, a first-order orientational transition was observed in the monolayer. This surface transition is shown to drive the reorientation of the bulk liquid crystal previously observed in these systems.

B. Jérôme;J. O'Brien;Y. Ouchi;C. Stanners;元壤 沈

Physical Review Letters

1993

Using an evanescent-wave ellipsometric technique, we have studied the wall-induced orientational order in the isotropic phase of 4-n-pentyl-4- cyanobiphenyl (5CB) at a silane-treated glass surface. Near the isotropic-nematic transition temperature TNI, a weak nematic layer appears at the interface with its thickness exhibiting a critical divergence as TTNI according to (T-TNI)-0.5. This observed pretransitional behavior is qualitatively at variance with the existing theories.

H. Hsiung;Th Rasing;元壤 沈

Physical Review Letters

1986

Laser-induced molecular reorientation and the associated Freedericksz transition in nematic films of 4-n-pentyl-4 cyanobiphenyl (5CB) with an air interface are studied. The results indicate that the 5CB-air interface actually behaves like a hard boundary in orienting the liquid-crystal molecules, but it allows an enhanced "backflow" in the medium through the orientation-flow coupling, which significantly speeds up the relaxation of molecular orientation. Theoretical analysis using the continuum theory is shown to be in good agreement with the experimental observations.

Ru Pin Pan;H. Hsiung;元壤 沈

Physical Review A

1987

We have measured the optical Kerr constant and the orientational relaxation times of seven p,p′-di-n-alkoxyazoxybenzene homologous compounds as functions of temperature in the isotropic phase. The observed critical behaviors near the isotropic-nematic transitions agree well with the predictions from the Landau-de Gennes model. Various characteristic parameters of the homologs are deduced from the experiment. Their variations with increase of methylene groups in the alkyl chain are discussed. Our results suggest that neither the mean-field theory of Maier and Saupe nor the Landau expansion of free energy is a good approximation for quantitative description at the isotropic-nematic transition.

E. G. Hanson;元壤 沈;G. K.L. Wong

Physical Review A

1976

Electrostrictive coefficients and optical Kerr constants for liquids are derived. Their relative importance in the self-focusing of a giant-pulse laser beam is discussed.

元壤 沈

Physics Letters

1966-3-1

We found two-magnon modes in the resonant Raman spectra of both our MnF2 sample and the purest MnF2 sample used by Rousseau et al., in the preceding paper. We present here raw spectra obtained from both samples.

Nabil M. Amer;Tai Chang Chiang;元壤 沈

Physical Review Letters

1976

The application of sum-frequency generation (SFG) vibrational spectroscopy to the study of surface alignment of polymer chains of polyvinyl alcohol by rubbing is discussed. The experiment involves the measurement of vibrational spectra of the CH₂ stretch modes with different input/output polarization combinations and various azimuthal orientations of the sample with respect to the incident plane. The results allow to deduce all the independent and nonvanishing nonlinear susceptibility components for the symmetric and antisymmetric stretch modes of CH₂.

元壤 沈

Conference Proceedings - Lasers and Electro-Optics Society Annual Meeting-LEOS

1999

UV photoluminescence (PL) from GaN thin films was observed by multiphoton excitation. The two-photon PL excitation spectrum near the band gap agrees with the theoretical two-photon-absorption spectrum. The pump-intensity dependence and the PL excitation spectrum in the infrared indicate the existence of midgap defect states around 1 eV above the valence band. This is confirmed by the PL excitation spectrum obtained with a two-color, two-photon excitation process.

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

Physical Review B - Condensed Matter and Materials Physics

1997

Using infrared-visible sum-frequency generation we have obtained the vibrational spectra of CH stretches of methanol molecules at the interface between methanol vapor and liquid. This is the first vibrational spectrum ever observed from a neat liquid surface. The measured polarization dependence and phase of the nonlinear susceptibility allow us to conclude that the surface methanol molecules are polar oriented with the CH3 group pointing away from the liquid with a very broad orientational distribution.

R. Superfine;J. Y. Huang;元壤 沈

Physical Review Letters

1991

Multiphonon resonant Raman scattering up to four phonons in GaSe and one- and two-phonon resonant Raman scattering in the mixed GaS _x Se_1-x crystals with x≤0.23 have been investigated. The results can be explained by a simple theory in which the dispersion of the resonance behavior is mainly dominated by resonances with the 1 s direct exciton state.

T. C. Chiang;J. Camassel;J. P. Voitchovsky;元壤 沈

Nuovo Cimento della Societa Italiana di Fisica B

1977-4