A theoretical formulation for optically active sum frequency generation (OA-SFG) from isotropic chiral solutions was proposed for molecules with a chiral side chain and an intrinsically achiral chromophore. Adapting an electron correlation model first proposed by Höhn and Weigang for linear optical activity, we presented a dynamic coupling model for OA-SFG near the electronic resonance of the achiral chromophore. As a demonstration, we used this model to explain the observed OA-SFG spectra of a series of amino acids near the electronic resonance of the intrinsically achiral carboxyl group. Our model shows that the nonlinear chiroptical response comes about by the through-space correlative electronic interactions between the chiral side chain and the achiral chromophore, and its magnitude is determined by the position and orientation of the bonds that make up the chiral side chain. Using the bond polarizability values in the literature and the conformations of amino acids obtained from calculation, we were able to reproduce the relative OA-SFG strength from a series of amino acids.

Na Ji;元壤 沈

Journal of the American Chemical Society

2005-9-21

The dynamics of laser-induced reorientation of LC molecules in a dye-doped LC medium was studied. Picosecond pump/probe measurements were used to monitor the time variation of the induced optical birefringence, the induced fluorescence, and the induced linear dichroism after the dye excitation. Comparing with the observed dynamics of induced birefringence in a pure LC medium, contributions due to dye molecules and LC molecules were separated.

Thai V. Truong;雷 徐;元壤 沈

Physical Review Letters

2003-5-16

Two-dimensional superconductors have attracted growing interest because of their scientific novelty, structural tunability, and useful properties. Studies of their magnetic responses, however, are often hampered by difficulties to grow large-size samples of high quality and uniformity. We report here an imaging method that employed NV^- centers in diamond as a sensor capable of mapping out the microwave magnetic field distribution on an ultrathin superconducting film of micron size. Measurements on a 33 nm thick film and a 125 nm thick bulklike film of Bi₂Sr₂CaCu₂O_8+δ revealed that the alternating current (ac) Meissner effect (or repulsion of ac magnetic field) set in at 78 and 91 K, respectively; the latter was the superconducting transition temperature of both films. The unusual ac magnetic response of the thin film presumably was due to thermally excited vortex-antivortex diffusive motion in the film. Spatial resolution of our ac magnetometer was limited by optical diffraction and the noise level was at 14 μT/Hz^1/2. The technique could be extended with better detection sensitivity to extract local ac conductivity/susceptibility of ultrathin or monolayer superconducting samples as well as ac magnetic responses of other two-dimensional exotic thin films of limited lateral size.

Ying Xu;Yijun Yu;Yuen Yung Hui;Yudan Su;Jun Cheng;Huan Cheng Chang;Yuanbo Zhang;元壤 沈;Chuanshan Tian

Nano Letters

2019-8-14

With amino acids as model systems, optically active sum frequency generation (OA-SFG) was used to probe the chirality of molecules with a chiral center and an intrinsically achiral chromophore in isotropic solution for the first time. Like that of circular dichroism (CD), the OA-SFG's near electronic resonance originates from the extrachromophoric chiral perturbation on the carboxyl chromophore. The difference in the relative strengths of OA-SFG and CD among different amino acids can be explained by the difference in the details of perturbations.

Na Ji;元壤 沈

Journal of the American Chemical Society

2004-11-24

Electrons moving in graphene behave as massless Dirac fermions, and they exhibit fascinating low-frequency electrical transport phenomena. Their dynamic response, however, is little known at frequencies above one terahertz (THz). Such knowledge is important not only for a deeper understanding of the Dirac electron quantum transport, but also for graphene applications in ultrahigh-speed THz electronics and infrared (IR) optoelectronics. In this paper, we report measurements of high-frequency conductivity of graphene from THz to mid-IR at different carrier concentrations. The conductivity exhibits Drude-like frequency dependence and increases dramatically at THz frequencies, but its absolute strength is lower than theoretical predictions. This anomalous reduction of free-electron oscillator strength is corroborated by corresponding changes in graphene interband transitions, as required by the sum rule.

Jason Horng;Chi Fan Chen;Baisong Geng;Caglar Girit;Yuanbo Zhang;Zhao Hao;Hans A. Bechtel;Michael Martin;Alex Zettl;Michael F. Crommie;元壤 沈;Feng Wang

Physical Review B - Condensed Matter and Materials Physics

2011-4-15

We demonstrate here, using α -quartz as an example, that IR-visible sum-frequency (SF) spectroscopy can be employed to probe zone-center optical phonons of crystals without inversion symmetry. Because only modes both IR and Raman active are allowed in the SF process, the observed phonon spectrum is greatly simplified. The intrinsic flexibility of SF spectroscopy also permits more accurate determination of relative magnitudes and signs of relevant Raman polarizabilities.

韡韬 刘;元壤 沈

Physical Review B - Condensed Matter and Materials Physics

2008-7-23

Surface plasmons (SPs) and phonon polaritons (PhPs) are two distinctive quasiparticles resulting from the strong coupling of photons with electrons and optical phonons, respectively. In this Letter, we investigate the interactions between one-dimensional (1D) plasmons in silver nanowires with two-dimensional (2D) surface phonon polaritons of the silicon carbide (SiC) substrate. Using near-field infrared spectroscopy of the silver nanowire-SiC heterostructure at wavelengths close to the phonon resonance of SiC, we observe that the 1D plasmon dispersion is strongly modified by the 2D phonon polaritons in SiC. In particular, we observe for the first time well-defined 1D plasmon oscillations with the plasmon wavelengths longer than the free-space photon wavelengths due to the 1D plasmon-2D phonon polariton coupling. Our work demonstrates that unusual polariton behavior can emerge from interactions between polariton excitons of different dimensionality, which can enable new ways to engineer plasmons in hybrid structures.

Trinity Joshi;Ji Hun Kang;Lili Jiang;Sheng Wang;Theron Tarigo;Tairu Lyu;Salman Kahn;Zhiwen Shi;元壤 沈;Michael F. Crommie;Feng Wang

Nano Letters

2017-6-14

Following our recent work [Phys. Chem. Chem. Phys. 20:5190–99 (2018)] that provided the means to unambigously define and extract the three water regions at any charged interface (solid–liquid and air–liquid alike), denoted the BIL (Binding Interfacial Layer), DL (Diffuse Layer) and Bulk, and how to calculate their associated non-linear Sum Frequency Generation Spectroscopy (SFG) χ²(ω) spectroscopic contributions from Density Functional Theory (DFT)-based ab initiomolecular dynamics simulations (DFT-MD/AIMD), we show here that theχ²DL(ω)signal arising from the DL water region carries a wealth of essential information on the microscopic and macroscopicproperties of interfaces. We show that theχ² _DL(ω)signal carries information on the surface potential and surface charge, the isoelectric point, EDL (Electric Double Layer) formation, and the relationship between a nominal electrolyte solution pH and surface hydroxylation state. This work is based on DFT-MD/AIMD simulations on a (0001) α–quartz–water interface and on the air–water interface, with various surface quartz hydroxylation states and various electrolyte concentrations. The conclusions drawn make use of the interplay between experiments and simulations. Most of the properties listed above can now be extracted from experimental χ² _DL(ω) alone with the protocols given in this work, or by making use of the interplay between experiments and simulations, as described in this work.

Simone Pezzotti;Daria Ruth Galimberti;元壤 沈;Marie Pierre Gaigeot

Minerals

2018-7-20

Infrared-visible doubly resonant sum-frequency spectroscopy has been successfully used to obtain optically active vibrational spectra of a chiral molecular monolayer. This establishes the technique as a unique tool for in situ probing of vibrational chirality.

M. A. Belkin;元壤 沈

OSA Trends in Optics and Photonics Series

2003

This chapter addresses what is known about the molecular structure at several aqueous-metal oxide interfaces, including near surface water. It indicates how improvements in this knowledge from new interface sensitive methodology aids our understanding of how these systems affect reaction progress. The chapter considers recent approaches via X-ray scattering and sum frequency vibrational spectroscopy (SFVS) methods that specifically probe the molecular structure of the oxide-water interface with bulk solution present, allowing pH and chemical variations to be studied in detail. The charging properties of aqueous-solid interfaces depend on the interaction of surface functional groups and other surface charge sources with overlying water molecules, although models that rely on the bulk dielectric properties of the solid substrate also exist. The general relationship between these charge sources has been developed as the electrical double layer (EDL) model. X-ray and SFVS techniques are very powerful for studies of oxide surfaces and interfaces.

元壤 沈;Glenn A. Waychunas

2013-7-29