Single-beam and enhanced two-beam second-harmonic generation from silicon nanocrystals by use of spatially inhomogeneous femtosecond pulses

Optical second-harmonic generation (SHG) is used as a noninvasive probe of the interfaces of Si nanocrystals (NCs) embedded uniformly in an SiO2 matrix. Measurements of the generated SH mode verify that the second-harmonic polarization has a nonlocal dipole form proportional to (E x Delta inverted) E that depends on inhomogeneities in the incident field E, as proposed in recent models based on a locally noncentrosymmetric dipolar response averaged over the spherical NC interfaces. A two-beam SHG geometry is found to enhance this polarization greatly compared to single-beam SHG, yielding strong signals useful for scanning, spectroscopy, and real-time monitoring. This configuration provides a general strategy for enhancing the second-order nonlinear response of centrosymmetric samples, as demonstrated here for both Si nanocomposites and their glass substrates.     

Propagation of intense laser pulses through inhomogeneous ionizinggas profiles

By simultaneously and self-consistently solving Maxwell's equations, the Ammosov-Delone-Krainov (ADK) field ionization equation, and the relativistic cold plasma equations, we have investigated the propagation of intense, ultrashort laser pulses through spatially inhomogeneous longitudinal gas gradients. Along with highly accurate calculations of the spatial and temporal beam profiles of the pulse at the end of various gradients, we have also determined simple scaling rules for the location of the vacuum-gas interface in order to minimize the pulse distortion at the focus. We show the benefits of using either preionized or low-Z gases, and we discuss the implications of this work for plasma-channel laser wakefield acceleration     

An investigation of the mode of sorption of inositol hexaphosphate to goethite

Adsorption of inositol hexaphosphate (IP(6)) on goethite has been studied as a function of pH and concentration, and by use of Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR). While adsorption was highest at low pH, a significant amount remained adsorbed above pH 10 where, in the absence of IP(6), the surface is expected to have a net negative charge. The adsorption isotherm at pH 5.5 indicated strong binding to the surface with each adsorbed species occupying about 2.5 nm(2). ATR-FTIR spectra of IP(6) solutions in the pH range from 2 to 12 were fitted with a single set of IR bands which were assigned primarily by analogy with phosphate spectra. From its variation in intensity with pH the band at 1040 cm(-1) was assigned to the effect of hydrogen bonding on the PO vibration. No additional bands were required to fit the spectra of IP(6) adsorbed to goethite, indicating that adsorption occurs by outer-sphere complexation in this system. At all pH values studied the band associated with hydrogen bonding was more intense for the adsorbed species than in solution at the corresponding pH indicating that hydrogen bonding plays an important role in binding IP(6) to goethite.     

A study on the stability of O2 on oxometalloporphyrins by the first principles calculations

The authors investigated the interaction of oxometalloporphyrins (MO(por))--specifically, MoO(por), WO(por), TiO(por), VO(por), and CrO(por)--with O(2) by using first principles calculations. MoO(por) and WO(por) undergo reactions with O(2); on the other hand, TiO(por), VO(por), and CrO(por) do not. Next, they compared the interaction of MoO(por) and WO(por) with O(2). Activation barriers for the reactions of MoO(por) and WO(por) with a side-on O(2) are small. For MoO(por)(O(2)), the activation barrier for the reverse reaction that liberates O(2) is also small; however, that for WO(por)(O(2)) is large. The experimental results that photoirradiation with visible light or heating of Mo (VI)O(tmp)(O(2)) regenerates Mo (VI)O(tmp) by liberating O(2) while W (VI)O(tmp)(O(2)) does not [J. Tachibana, T. Imamura, and Y. Sasaki, Bull. Chem. Soc. Jpn. 71, 363 (1998)] are explained by the difference in activation barriers of the reverse reactions. This means that bonds formed between the W atom and O(2) are stronger than those between the Mo atom and O(2). The bond strengths can be explained by differences in the energy levels between the highest occupied molecular orbital of MoO(por) and WO(por), which are mainly formed from the a orbitals of the central metal atom and pi(*) orbitals of O(2).     

Localized, targeted, and sustained siRNA delivery

Short interfering RNA (siRNA) functions directly in the cytoplasm, where it is assembled into an RNA-induced silencing complex (RISC). The localized delivery of siRNA to a specific site in vivo is highly challenging. There are many disease states in which a systemic effect of RNAi may be desirable; some examples include non-localized cancers, HIV, neurodegenerative diseases, respiratory viruses, and heart and vascular disease. In this Concept, we will focus on the localized delivery of siRNA to a target site using various delivery modalities. In certain tissues, such as the eye, central nervous system and lung, it has been demonstrated that a simple injection of naked siRNA will silence gene expression specifically in that tissue. To achieve local gene silencing in other tissues, a variety of approaches have been pursued to help stabilize the siRNA and facilitate uptake; they include chemical modification of the siRNA or complexation within liposomes or polymers to form nanoparticles. Recently, the use of macroscopic biomaterial scaffolds for siRNA delivery has been reported, and although there is still significant work to be done in this area to optimize the delivery systems, it is an important area of research that offers the potential for having great impact on the field of siRNA delivery.     

The ring as a torsion-free cover

LetR be an integral domain andI a non-zero ideal ofR. The canonical mapR→R/I is called atorsion-free cover ofR/I if everyR-homomorphism from a torsion-freeR-module intoR/I can be factored throughR. The main result of this paper is thatR→R/I is a torsion-free cover if and only ifR is complete in theR-topology andI is an ideal of injective dimension 1. In this caseI is contained in the Jacobson radical ofR. And if Λ is the endomorphism ring ofI, then Λ is a quasi-local domain. IfI is a flatR-module, thenQ→Q/Λ is a torsion-free cover, whereQ is the quotient field ofR. And thenQ/Λ is an indecomposable injectiveR (and Λ) module. Special results are obtained ifR is a Noetherian domain or a Prüfer domain.