Nanosecond dynamics of ferroelectric/dielectric superlattices

The nanosecond response of a PbTiO(3)/SrTiO(3) ferroelectric/dielectric superlattice to applied electric fields is closely linked to the dynamics of striped domains of the remnant polarization. The intensity of domain satellite reflections observed with time-resolved x-ray microdiffraction decays in 5-100 ns depending on the magnitude of the electric field. The piezoelectric response of the superlattice within stripe domains is strongly suppressed due to electromechanical clamping between adjacent regions of opposite polarization. Regions of the superlattice that have been switched into a uniform polarization state by the applied electric field, however, exhibit piezoelectricity during the course of the switching process. We propose a switching model different from previous models of the switching of superlattices, based instead on a spatially heterogeneous transformation between striped and uniform polarization states.     

The Three-Wave Resonant Interaction Equations: Spectral and Numerical Methods

The spectral theory of the integrable partial differential equations which model the resonant interaction of three waves is considered with the purpose of numerically solving the direct spectral problem for both vanishing and non vanishing boundary values. Methods of computing both the continuum spectrum data and the discrete spectrum eigenvalues are given together with examples of such computations. The explicit spectral representation of the Manley-Rowe invariants is also displayed.     

Analytic soliton solutions of cubic‐quintic Ginzburg‐Landau equation with variable nonlinearity and spectral filtering in fiber lasers

In fiber lasers, the study of the cubic‐quintic complex Ginzburg‐Landau equations (CGLE) has attracted much attention. In this paper, four families (kink solitons, gray solitons, Y‐type solitons and combined solitons) of exact soliton solutions for the variable‐coefficient cubic‐quintic CGLE are obtained via the modified Hirota method. Appropriate parameters are chosen to investigate the properties of solitons. The influences of nonlinearity and spectral filtering effect are discussed in these obtained exact soliton solutions, respectively. Methods to amplify the amplitude and compress the width of solitons are put forward. Numerical simulation with split‐step Fourier method and fourth‐order Runge‐Kutta algorithm are carried out to validate some of the analytic results. Transformation from the variable‐coefficient cubic‐quintic CGLE to the constant coefficients one is proposed. The results obtained may have certain applications in soliton control in fiber lasers, and may have guiding value in experiments in the future.

     

Nonlinear optical characteristics of monolayer MoSe2

In this study, we utilized picosecond pulses from an Nd:YAG laser to investigate the nonlinear optical characteristics of monolayer MoSe₂. Two‐step growth involving the selenization of pulsed‐laser‐deposited MoO₃ film was employed to yield the MoSe₂ monolayer on a SiO₂/Si substrate. Raman scattering, photoluminescence (PL) spectroscopy, and atomic force microscopy verified the high optical quality of the monolayer. The second‐order susceptibility χ⁽²⁾ was calculated to be ～50 pm V^?1 at the second harmonic wavelength ～810 nm, which is near the optical gap of the monolayer. Interestingly, our wavelength‐dependent second harmonic scan can identify the bound excitonic states including negatively charged excitons much more efficiently, compared with the PL method at room temperature. Additionally, the MoSe₂ monolayer exhibits a strong laser‐induced damage threshold ～16 GW cm^?2 under picosecond‐pulse excitation_. Our findings suggest that monolayer MoSe₂ can be considered as a promising candidate for high‐power, thin‐film‐based nonlinear optical devices and applications.     

Parametric Study of Nonplanar Dust Acoustic Solitons in Two‐Dimensional Superthermal Dusty Plasmas

The nonlinear properties of two dimensional low‐frequency electrostatic excitations of charged dust particles (or defects) are studied in a collisionless, unmagnetized dusty plasma. A fully ionized three‐component model plasma consisting of kappa distributed electrons, Maxwellian ions, and negatively charged massive dust grains is considered. In this regard, the well known reductive perturbation technique is used to the hydrodynamical equations and the Poisson equation, obtaining the cylindrical Kadomtsev–Petviashvili (CKP) equation. A parametric investigation indicates that the structural characteristics of these nonlinear excitations (width, amplitude) are significantly affected by the plasma nonthermality as well as by the relevant plasma parameters, such as dust concentration and dust temperature. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Tuning protonation states of tripelennamine antihistamines by cucurbit[7]uril

The formation of host–guest complexes of cucurbit[7]uril (CB7) with the antihistamine drug tripelennamine (TRP) was studied by optical and NMR spectroscopy. The experimental and computational results are consistent with inclusion of a single TRP molecule in CB7. Addition of CB7 has tuned drug protonation states associated with (de)protonation of the ethyldimethylammonium and exocyclic nitrogens with an increase in the associated pK_a values by 1.5 and 2.5 units, respectively. The incorporation of antihistamines drugs such as TRP in cucurbiturils could be utilized for switching their capability for selective binding to histamine H₁‐receptors, in the future. Copyright © 2015 John Wiley & Sons, Ltd.     

Synthesis of Blue‐, Green‐, Yellow‐, and Red‐Emitting Graphene‐Quantum‐Dot‐Based Nanomaterials with Excitation‐Independent Emission

A one‐pot method is described for the preparation of graphene quantum dots/graphene oxide (GQDs/GO) hybrid composites with emission in the visible region, through heteroatom doping and hydroxyl‐radical‐induced decomposition of GO. The NH₄OH‐ and thiourea‐mediated dissociation of H₂O₂ produces hydroxyl radicals. Treatment of GO with hydroxyl radicals results in the production of small‐sized GO sheets and GQDs, which self‐assemble to form GQDs/GO through strong π–π interactions. For example, the reaction of GO with a mixture of NH₄OH and H₂O₂ for 40, 120, and 270 min generates yellow‐emitting GQDs/GO (Y‐GQDs/GO), green‐emitting GQDs/GO, and blue‐emitting GQDs, while red‐emitting GQDs/GO (R‐GQDs/GO) are prepared by incubating GO with a mixture of thiourea and H₂O₂. From the analysis of these four GQD‐based nanomaterials by transmission electron microscopy, atomic force microscopy, and fluorescence lifetime spectroscopy, it is found that this tunable fluorescence wavelength results from the differences in particle size. All four GQD‐based nanomaterials exhibit moderate quantum yields (1–10%), nanosecond fluorescence lifetimes, and excitation‐independent emissions. Except for R‐GQDs/GO, the other three GQD‐based nanomaterials are stable in a high‐concentration salt solution (e.g., 1.6 m NaCl) and under high‐power irradiation, enabling the sensitive (high‐temperature resolution and large activation energy) and reversible detection of temperature change. It is further demonstrated that Y‐GQD/GO can be used to image HeLa cells.     

Improved growth of solution‐deposited thin films on polycrystalline Cu(In,Ga)Se2

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se₂ (CIGS) solar cells. As recently reported, a non‐contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as‐grown CIGS layers, these buffer lay‐ ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i‐ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air‐annealed CIGS films with improved buffer coverage resulted in higher power‐conversion efficiencies, as compared with the devices containing as‐grown CIGS layers. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)