Sum-frequency vibrational spectroscopy, with the help of surface pressure-area (π-A) isotherm, was used to study lipid Langmuir monolayers composed of molecules with positively and negatively charged headgroups as well as a 1:1 neutral mixture of the two. The spectral profiles of the CHx stretch vibrations are similar for all monolayers in the liquid-condensed (LC) phase. They suggest a monolayer structure of closely packed alkyl chains that are nearly all-trans and well oriented along the surface normal. In the liquid-expanded (LE) phase, the spectra of all monolayers appear characteristic of loosely packed chains with significant gauche defects. The OH stretch spectra of interfacial water for both positively and negatively charged monolayers are significantly enhanced in comparison with a neutral water interface, but the phase measurement of SFVS indicates that OH in the two cases points toward the bulk and the interface, respectively. The enhancement results mainly from surface-field-induced polar ordering of interfacial water molecules. For a charge-neutral monolayer composed of an equal number of positively and negatively charged lipid molecules, no such enhancement is observed. This mixed monolayer exhibits a wide range of LC/LE coexistence region extended to very low surface pressure and its CHx spectral profile in the coexistence region resembles that of the LC phase. This result suggests that in the LC/LE coexistence region, the mixed monolayer consists of coexisting LC and LE patches in which oppositely charged lipid molecules are homogeneously mixed and dispersed.
Woongmo Sung;Sangjun Seok;Doseok Kim;传山 田;元壤 沈
Sum-frequency vibrational spectroscopy (SFVS) has been used to probe the chirality of a polymer thin film. The polymer used has a π-electron conjugated structure in the main chain, and a chiral center in the side chains that is close to the main chain, giving rise to the helically twisted main chain. The vibrational modes probed are stretches associated with the main chain. As expected for chiral responses, we find that the chiral SFVS spectra are identical for the two enantiomers, and the signal vanishes for the racemic polymer. We also confirm experimentally that the chiral nonlinear susceptibilities for SFVS for the two enantiomers have opposite signs.
Masahito Oh-E;Hiroshi Yokoyama;Shinichi Yorozuya;Kazuo Akagi;M. A. Belkin;元壤 沈
Molecular Crystals and Liquid Crystals
(Figure Presented) Phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) has been used to probe the isotopically diluted water/vapor interfaces in the spectral regions of OD (2200-2800 cm -1) and OH (3000-3800 cm -1) stretches. The experimentally measured Im X S (2) spectra, where X S (2) is the surface nonlinear susceptibility, permit direct characterization of resonances of the interfaces. The X S (2) spectrum of the HDO/vapor interface that is intrinsically simpler to analyze can be deduced from the result of isotopic dilution. It exhibits in the bonded-OH region a broad band comprising two parts with opposite signs, in contrast to those deduced earlier from fitting of the |X S (2)| 2 spectra and those calculated by MD simulation, but consistent with that obtained for the H 2O/vapor interface.
传山 田;元壤 沈
Journal of the American Chemical Society
A sum-frequency generation (SFG) microscope that is sensitive toward molecular chirality was demonstrated for the first time. Optically active images of chiral 1,1′-bi-2-naphthol solutions were obtained with submicron spatial resolution. Three-dimensional sectioning capability of our microscope was also demonstrated. This optically active SFG microscopy can potentially become a powerful imaging technique for biological samples.
Na Ji;Kai Zhang;Haw Yang;元壤 沈
Journal of the American Chemical Society
Using an optical Kerr effect experiment, the rotation of a series of cyanobiphenyl liquid crystals about their long axis in SmA phase and their relaxation were measured for the first time on picosecond and femtosecond time scales. The fast observed optical nonlinearity is related to cyanobiphenyl head rotation and is bigger compared to benzene.
E. Freysz;L. Sarger;小威 庄;T. Moses;元壤 沈
Despite unique and fascinating capability in subwavelength optics, active control of optical plasmon resonance has been hampered by drastically weak optical response of free carriers at optical frequency. We demonstrate efficient control of optical plasmon resonance in gold nanorod with graphene by electrical gating.
Jonghwan Kim;Hyungmok Son;David J. Cho;Baisong Geng;Will Regan;Sufei Shi;Kwanpyo Kim;Alex Zettl;元壤 沈;Feng Wang
We show by pump-probe spectroscopy that the optical response of a fishnet metamaterial can be modulated on the femtosecond time scale. The modulation dynamics is dominated by pump-induced changes in the constituting dielectric medium, but the strength of modulation is dramatically enhanced through the plasmon resonance. The pump-induced spectral responses of the metamaterial provide understanding on how the resonance is modified by pump excitation. Our study suggests that metamaterials can be used as high-speed amplitude/phase modulators with terahertz-bandwidth.
David J. Cho;Wei Wu;Ekaterina Ponizovskaya;Pratik Chaturvedi;Alexander M. Bratkovsky;Shih Yuan Wang;翔 张;Feng Wang;元壤 沈
We demonstrate experimentally that visible-visible sum-frequency generation in the bulk of a chiral liquid is observable near electronic resonant transitions. Although the process is electric dipole allowed, it is rather weak because the orientational average over molecules effectively reduces the bulk chiral nonlinearity.
M. A. Belkin;S. H. Han;X. Wei;元壤 沈
Physical Review Letters
Using sum-frequency vibrational spectroscopy, we found that water structure at nanoporous silica/water interfaces depended on the nanoporous film structure. For a periodic, self-assembled nanoporous film with monosized 2 nm pores occupying 20% of the top surface area, the surface vibrational spectrum was dominated by water in contact with silica, bare or covered by silane, at the top surface. It resembled the spectral characteristic of the hydrophilic water/silica or the hydrophobic water/silane interface. For a fractal nanoporous film with pores ranging from 5 to 50 nm in size occupying 90% of the top surface, the spectrum for a trimethyl silane-coated superhydrophobic porous film resembled largely that of a water/air interface. Only when the silane was completely removed would the spectrum revert to that characteristic of a hydrophilic water/silica interface. The surface charging behaviors of the bare nanoporous films in water with different pH were monitored by spectroscopic measurements and atomic force microscopy force measurements. The point of zero charge for the periodic porous film is around pH 2, similar to that of the flat silica surface. The point of zero charge could only be determined to be pH<6 for the fractal porous film because the thin fractal solid network limited the amount of surface charge and therefore, the accuracy of the measurements.
Luning Zhang;Seema Singh;传山 田;元壤 沈;Yan Wu;Mark A. Shannon;C. Jeffery Brinker
Journal of Chemical Physics
Sum-frequency vibrational spectroscopy was used to assess water molecules ordering and orientation at the water/α-quartz interface. A special scheme allowed suppression of quartz bulk signal and measurement of the amplitude and phase of the nonlinear susceptibility.
Victor Ostroverkhov;Glenn A. Waychunas;元壤 沈
OSA Trends in Optics and Photonics Series