Y. R. Shen

Bio: Y. R. Shen is an academic researcher from University of California, Berkeley. The author has contributed to research in topics: Liquid crystal & Monolayer. The author has an hindex of 98, co-authored 476 publications receiving 37313 citations. Previous affiliations of Y. R. Shen include University of Paris & Harvard University.

Optical second-harmonic generation from magnetized surfaces.

Abstract: We propose optical second-harmonic generation as a means to probe surface magnetization. It is shown that surface magnetization can induce a number of nonlinear susceptibility elements that would vanish otherwise. They are presented for the (001), (110), and (111) surfaces of a fcc centrosymmetric crystal. An order-of-magnitude estimate, using the microscopic expression of the nonlinear susceptibility, suggests that these induced elements are detectable by optical second-harmonic generation with appropriate polarization combinations. The second-harmonic signals from magnetized and nonmagnetized surfaces should exhibit characteristically different rotational anisotropy.

Surface vibrational spectroscopic study of surface melting of ice.

Abstract: Surface melting on the (0001) face of hexagonal ice ( I(h)) was studied by sum-frequency vibrational spectroscopy in the OH stretch frequency range. The degree of orientational order of the dangling OH bonds at the surface was measured as a function of temperature. Disordering sets in around 200 K and increases dramatically with temperature. The results show that the disordered (quasiliquid) layer on ice is structurally different from normal liquid water.

Structure and charging of hydrophobic material/water interfaces studied by phase-sensitive sum-frequency vibrational spectroscopy

Abstract: We have studied the hydrophobic water/octadecyltrichlorosilane (OTS) interface by using the phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS), and we obtained detailed structural information of the interface at the molecular level. Excess ions emerging at the interface were detected by changes of the surface vibrational spectrum induced by the surface field created by the excess ions. Both hydronium (H3O+) and hydroxide (OH−) ions were found to adsorb at the interface, and so did other negative ions such as Cl−. By varying the ion concentrations in the bulk water, their adsorption isotherms were measured. It was seen that among the three, OH− has the highest adsorption energy, and H3O+ has the lowest; OH− also has the highest saturation coverage, and Cl− has the lowest. The result shows that even the neat water/OTS interface is not neutral, but charged with OH− ions. The result also explains the surprising observation that the isoelectric point appeared at ∼3.0 when HCl was used to decrease the pH starting from neat water.

Investigation of surface-induced alignment of liquid-crystal molecules by optical second-harmonic generation

Abstract: We apply the technique of optical second-harmonic generation to study homogeneously aligned liquid-crystal cells. The surface dipole sensitivity of the technique made it possible to study the monolayer in the absence and presence of a bulk of liquid crystal. By comparing the monolayer orientational distribution functions of three surface treatments (rubbed polymer-coated substrates, rubbed surfactant-coated substrates, and substrates made from oblique evaporation of ${\mathrm{SiO}}_{\mathit{x}}$ film), we find that two different surface-originated mechanisms are effective in aligning liquid-crystal films. For rubbed polymer samples, it is shown that a short-range molecular interaction is responsible for alignment of the first monolayer, which then aligns the bulk via an epitaxylike interaction. Results on polymers with various structures and compositions and rubbed with a variety of rubbing strengths are presented. For the other surface treatments, the first monolayer is isotropically distributed, indicating that a bulk elastic interaction is responsible for the bulk alignment.

Generation of Far‐Infrared Radiation by Picosecond Light Pulses in LiNbO3

Abstract: We have observed far‐infrared radiation generated by picosecond pulses in LiNbO3 with several different phase‐matching conditions. The output spectra, analyzed by a far‐infrared Michelson interferometer and by a Fabry‐Perot interferometer, agree well with theoretical calculations. The laser pulsewidth deduced from these measurements was about 2 psec in comparison with 5 psec obtained from two‐photon fluorescence measurements.

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Abstract: Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

The Optical Properties of Metal Nanoparticles: The Influence of Size, Shape, and Dielectric Environment

Abstract: The optical properties of metal nanoparticles have long been of interest in physical chemistry, starting with Faraday's investigations of colloidal gold in the middle 1800s. More recently, new lithographic techniques as well as improvements to classical wet chemistry methods have made it possible to synthesize noble metal nanoparticles with a wide range of sizes, shapes, and dielectric environments. In this feature article, we describe recent progress in the theory of nanoparticle optical properties, particularly methods for solving Maxwell's equations for light scattering from particles of arbitrary shape in a complex environment. Included is a description of the qualitative features of dipole and quadrupole plasmon resonances for spherical particles; a discussion of analytical and numerical methods for calculating extinction and scattering cross-sections, local fields, and other optical properties for nonspherical particles; and a survey of applications to problems of recent interest involving triangula...

Graphene photonics and optoelectronics

Abstract: The richness of optical and electronic properties of graphene attracts enormous interest. Graphene has high mobility and optical transparency, in addition to flexibility, robustness and environmental stability. So far, the main focus has been on fundamental physics and electronic devices. However, we believe its true potential lies in photonics and optoelectronics, where the combination of its unique optical and electronic properties can be fully exploited, even in the absence of a bandgap, and the linear dispersion of the Dirac electrons enables ultrawideband tunability. The rise of graphene in photonics and optoelectronics is shown by several recent results, ranging from solar cells and light-emitting devices to touch screens, photodetectors and ultrafast lasers. Here we review the state-of-the-art in this emerging field.