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Interface-specific hydrogen (H)–bonding network of water directly controls the energy transfer and chemical reaction pathway at many charged aqueous interfaces, yet to characterize these bonded water layer structures remains a challenge. We now develop a sum-frequency spectroscopic scheme with varying photon momenta as an all-optic solution for retrieving the vibrational spectra of the bonded water layer and the ion diffuse layer and, hence, microscopic structural and charging information about an interface. Application of the method to a model surfactant-water interface reveals a hidden weakly donor H-bonded water species, suggesting an asymmetric hydration-shell structure of fully solvated surfactant headgroups. In another application to a zwitterionic phosphatidylcholine lipid monolayer–water interface, we find a highly polarized bonded water layer structure associating to the phosphatidylcholine headgroup, while the diffuse layer contribution is experimentally proven to be negligible. Our all-optic method offers an in situ microscopic probe of electrochemical and biological interfaces and the route toward future imaging and ultrafast dynamics studies.

We develop and verify a phase-sensitive second harmonic generation spectroscopic scheme that allows for direct determination of the absolute surface charge density and surface potential of a water interface without need of prior interfacial information. The method relies on selective probing of surface-field-induced reorientation order of water molecules in the electrical double layer and is, hence, independent of the interfacial molecular bonding structure. Application of this technique to a mixed surfactant monolayer on water suggests the manifest effect of the chain-chain interactions among the monolayer on adsorption of soluble ionic surfactants. We also deduce the third-order nonlinear susceptibility of bulk water and prove its applicability to analysis of charges of various water interfaces. In addition, we show that the Debye-H\"uckle theory should be avoided in the spectroscopic analysis for its potential significant error, as evidenced experimentally and theoretically., Comment: 19 manuscript pages, four figures, 14 pages supporting information included

Objective: Previous study has demonstrated that EphA2 is a biomarker of mesenchymal stem cells (MSCs) from human placenta or umbilical cord and is able to distinguish MSCs from fibroblasts. In this study, we further examine the potential efficacy of EphA2+ human umbilical cord-derived MSCs (hUC-MSCs). Materials and methods: MSCs specific markers, EphA2 and CD146 expression on the surface of hUC-MSCs were determined by flow cytometry analysis. Quantitative real time polymerase chain reaction was used to examine pro-fibrotic gene expression of TGF-β1-stimulated lung fibroblast (MRC-5 cells). On the other hand, ELISA was used to analyze the content of pro-inflammatory cytokines (TNF-ɑ; and IP-10) in the LPS-activated macrophages culture supernatant. Results: The pro-fibrotic gene (TGF-β1, CTGF, fibronectin, collagen I and TIMP-1) expression in TGF-β1-activated MRC-5 cells and the pro-inflammatory cytokines (TNF-ɑ and IP-10) in the LPS-activated macrophages culture supernatant were both attenuated when in present of EphA2+ hUC-MSCs. Moreover, once EphA2+ hUC-MSCs treated with prostaglandin E2 specific inhibitor NS-398, both anti-fibrotic and anti-inflammatory effects of EphA2+ hUC-MSCs were abolished. Conclusion: EphA2+ hUC-MSCs possess immunomodulatory and anti-fibrotic properties, and PGE2 plays an important role in these activities. This implies that EphA2+ hUC-MSCs have potentially effectiveness for treatment of acute inflammatory and chronic fibrotic lung diseases. Keywords: Anti-fibrosis, EphA2, Immunomodulatory, Mesenchymal stem cells, Prostaglandin E2

We have utilized ultrafast optical spectroscopy to study carrier dynamics in slightly underdoped (BaK)Fe2As2 crystals without magnetic transition. The photoelastic signals due to coherent acoustic phonons have been quantitatively investigated. According to our temperature-dependent results, we found that the relaxation component of superconducting quasiparticles persisted from the superconducting state up to at least 70 K in the normal state. Our findings suggest that the pseudogaplike feature in the normal state is possibly the precursor of superconductivity. We also highlight that the pseudogap feature of K-doped BaFe2As2 is different from that of other iron-based superconductors, including Co-doped or P-doped BaFe2As2.

Charged water interfaces are ubiquitous and responsible for many important processes in nature and modern technologies, such as protein folding, electrochemistry, and photocatalysis. They appear in the form of an electric double layer that can be divided into two sub-layers. One is the so-called Stern layer composed of one to two hardly mobile, hydrogen-bonded, water monolayers next to the charged plane. The other is the diffuse layer, in which ions assuming the Poisson-Boltzmann distribution set up a dc field distribution. Being directly associated with the charged surface, the Stern layer directly controls the microscopic energy transfer and chemical reaction pathway at the interface. However, despite extensive studies on charged water interfaces, current knowledge on the microscopic structure of the Stern layer is still extremely limited. The difficulty lies in the inability of existing techniques to selectively extract structural information about the Stern layer in the presence of the diffuse layer. We now have developed a sum-frequency spectroscopy method that allows us to obtain the vibrational spectrum, and hence the microscopic structural information, of a Stern layer. Application of the method to a prototype lipid (fatty acid)-water interface reveals significant variation of its Stern layer structure upon deprotonation of the lipid headgroup. The measurement also yields a spectrum that characterizes the dc-field-induced sum-frequency generation from bulk water in general, and can help to deduce vibrational spectra of the Stern layer of other charged water interfaces. This unique ability of our method provides opportunities to gain better microscopic understanding of properties and functionality of charged water interfaces.

We demonstrate a beam-scanning nonlinear light endoscope based on a flexible fiber bundle. Excited with a femtosecond Cr:Forsterite laser, the degradation in multiphoton multiharmonic excitation efficiency due to the pulse-broadening effect is significantly reduced without utilizing any external devices. The system resolution has been characterized to be 5.4 μm in the two-photon fluorescence endoscope, limited by the sampling theory. Finally, several image examples have been given. Microsc. Res. Tech., 2008. © 2008 Wiley-Liss, Inc.

Despite the numerous devoted studies, water at solid interfaces remains puzzling. An ongoing debate concerns the nature of interfacial water at a hydrophilic surface, whether it is more solid-like, ice-like, or liquid-like. To answer this question, a complete picture of the distribution of the water molecule structure and molecular interactions has to be obtained in a non-invasive way and on an ultrafast time scale. We developed a new experimental technique that extends the classical acoustic technique to the molecular level. Using nanoacoustic waves with a femtosecond pulsewidth and an angstrom resolution to noninvasively diagnose the hydration structure distribution at ambient solid/water interface, we performed a complete mapping of the viscoelastic properties and of the density in the whole interfacial water region at hydrophilic surfaces. Our results suggest that water in the interfacial region possesses mixed properties and that the different pictures obtained up to now can be unified. Moreover, we discuss the effect of the interfacial water structure on the abnormal thermal transport properties of solid/liquid interfaces.

We utilize steady-state and transient optical spectroscopies to examine the responses of nonthermal quasiparticles with respect to orbital modifications in normal-state iron-chalcogenide superconductors. The dynamics shows the emergence of gaplike quasiparticles (associated with a $\ensuremath{\sim}36\text{ }\text{ }\mathrm{meV}$ energy gap) with a coincident transfer of the optical spectral weight in the visible range, at temperatures above the structural distortion. Our observations suggest that opening of the high-temperature gap and the lattice symmetry breaking are possibly driven by short-range orbital and/or charge orders, implicating a close correlation between electronic nematicity and precursor order in iron-based superconductors.

We report a direct determination of the specular scattering probability of acoustic phonons at a crystal boundary by observing the escape of incident coherent phonons from the coherent state during reflection. In the sub-THz frequency range where the phonon wavelength is much longer than the lattice constant, the acoustic phonon-interface interaction is found to agree well with the macroscopic theory on wave scattering from rough surfaces. This examination thus quantitatively verifies the dominant role of atomic-scale corrugations in the Kapitza anomaly observed at 1-10 K and further opens a new path to nondestructively estimate subnanoscale roughness of buried interfaces.

Received 18 March 2009; revised manuscript received 17 September 2009; published 6 November 2009 Using piezoelectric wurtzite semiconductor with accurate control of the crystal cut, we investigate the photogeneration of coherent shear acoustic phonons through anisotropic piezoelectric coupling. Theoretical study suggests the dominant contribution of the piezoelectric effect to the shear phonon generation in a-plane wurtzite GaN, which is also confirmed by the picosecond ultrasonic experiments with different dopant and excitation conditions. Piezoelectric transduction thus explains the observed remarkable coherent shear phonon signal. The influence of the ultrafast screening of surface electric field by photocarriers on the intensity and waveform of the generated shear acoustic pulse are discussed.

International audience; Taking advantage of the specific core-shell charge separation structure in the CdSe/CdTe core-shell Type-II quantum dots (QDs), we experimentally observed the resonant-enhanced dipolar interaction between millimeter-wave (MMW) photons and their corresponding (l = 1) confined acoustic phonons. With proper choice of size, the absorption band can be tuned to desired frequency of MMW imaging. Exploiting this characteristic absorption, in a fiber-scanned MMW imaging system, we demonstrated the feasibility of CdSe/CdTe QDs as the contrast agents of MMW imaging.

The ultrafast quasiparticle dynamics of Fe1+ySe1?xTex single crystals were investigated by dual-color transient reflectivity measurements (?R/R) from 4.3 to 290?K. The electron?phonon coupling strength ??(=0.16?0.01) and the temperature-dependent energy of longitudinal-acoustic phonons were, respectively, obtained from the relaxation time of a fast component and the period of an oscillation component in ?R/R. Such a small ? demonstrates the unconventional origin of superconductivity in FeSe. Moreover, the temperature-dependent ?R/R exhibits anomalous changes at Tc, 90?K and 230?K, unambiguously revealing the phase transition as well as the phonon softening via the magnetoelastic effect.

We demonstrated a nanoscaled artificial phononic crystal composed of three-dimensionally ordered quantum dots (QDs) with functional acoustic properties. Femtosecond ultrasonic technique is used to investigate the lattice dynamics of this phononic nanocrystal. The measurement results indicate that three-dimensional ordering and uniformity of the QDs are important factors influencing the observed acoustic resonance at the forbidden bands. For well-arranged QDs, noticeable features of the phononic band gap and the associated phonon cavity mode can be found, while this nanocrystal also serves as an effective acoustic medium, or acoustic meta material, for low-frequency acoustic phonons.

We report that the electron cooling time in indium nitride can be as fast as sub-100 femtosecond at low electron concentration (

We demonstrate electrical control of the optical sensitivity function in multiple quantum wells (MQWs) for nanoacoustic wave detection. This is realized by bias controlling the quantized level and the quasi-Fermi level of carrier-populated InGaN/GaN MQWs. Experimentally, a strongly bias-dependent optical sensitivity was observed when the optical probe transition was near the quasi-Fermi level, which agrees well with the theoretical prediction.

Ultrafast carrier thermalization in n-type indium nitride (InN) with an electron concentration of 3.8×1018 cm−3 was investigated by femtosecond transient transmission measurements at room temperature with different wavelengths. An extremely fast carrier external thermalization time on the order of 400 fs was observed, which is much faster than all previous reports. This observed femtosecond thermalization time is consistent with a prediction based on a Coulomb screening effect. Through wavelength dependent and power dependent studies, even with a 400 fs thermalization time, we did not observe any evidence of the existence of the hot phonon effect, which agrees with a recent report that a longitudinal optical phonon lifetime could be shorter than 300 fs in specific InN samples.

Sub-THz coherent acoustic phonon coupling between solid and liquid water was studied by the femtosecond pump-probe technique. We used InGaN/GaN heterostructures to generate narrow-band coherent acoustic phonons at frequencies up to 1.4 THz. The measured ratio of reflectivities at GaN-water interfaces to that at GaN-air interfaces was found to strongly decrease and exhibit an abnormal oscillation with the increase in phonon frequency. The possible mechanisms resulting in the characteristics of experimental observations are briefly discussed in this report.

The authors demonstrated that the piezoelectric superlattice, can serve as narrow-band detectors for propagating coherent longitudinal acoustic phonons at multiple frequencies corresponding to the spatial frequency of the superlattice and its higher harmonics, and its detection bandwidth is determined by the total structure width. By optically exciting a broadband propagating longitudinal acoustic pulse from a thin Ni film, the authors studied the acoustic spectral sensitivity function of a ten-period In0.12Ga0.88N∕GaN multiple quantum well. Because the barriers (19nm) and wells (3.6nm) are of different widths, the second detection band, corresponding to the second harmonic of the fundamental frequency, can be resolved.

Picosecond ultrasonic experiments were performed to generate and detect acoustic pulses in piezoelectric (111) epilayers of InxGa(1−x)As, and the longitudinal sound velocities in the [111] direction of InxGa(1−x)As were thus directly determined by controlling the indium content x. We found that the sound velocity exhibits an obvious bowing with different alloy compositions and can be described as a quadratic function of VL(x)=458x2−1451x+5414. This result supports a nonlinear compositional dependence of the elastic constants of InxGa(1−x)As and indicates that the linear-interpolation approximation is inapplicable for estimating sound velocity in this specific case.

We demonstrate the generation of picosecond acoustic pulses using a piezoelectric-semiconductor-based p‐n junction structure. This p‐n junction picosecond ultrasonic experiment confirms that the piezoelectric effect dominates the thermal expansion and deformation-potential coupling in the generation of picosecond acoustic pulses. The characteristics of the p‐n initiated acoustic pulses are determined by the width and the field strength inside the depletion region. Our study indicates the future possibility to electrically control the acoustic pulse characteristics if we could apply an external bias to modulate the depletion region width.