Nonlinear diffraction of light beams propagating in photorefractive media with embedded reflecting wire

The theory of nonlinear diffraction of intensive light beams propagating through photorefractive media is developed. Diffraction occurs on a reflecting wire embedded in the nonlinear medium at a relatively small angle with respect to the direction of the beam propagation. It is shown that this process is analogous to the generation of waves by a flow of a superfluid past an obstacle. The ""equation of state"" of such a superfluid is determined by the nonlinear properties of the medium. On the basis of this hydrodynamic analogy, the notion of the ""Mach number"" is introduced where the transverse component of the wave vector plays the role of the fluid velocity. It is found that the Mach cone separates two regions of the diffraction pattern: inside the Mach cone oblique dark solitons are generated and outside the Mach cone the region of ""optical ship waves"" (the wave pattern formed by a two-dimensional packet of linear waves) is situated. Analytical theory of the ""optical ship waves"" is developed and two-dimensional dark soliton solutions of the generalized two-dimensional nonlinear Schrodinger equation describing the light beam propagation are found. Stability of dark solitons with respect to their decay into vortices is studied and it is shown that they are stable for large enough values of the Mach number.

Formation of atomic carbon chains from graphene nanoribbons

The formation of one-dimensional carbon chains from graphene nanoribbons is investigated using ab initio molecular dynamics. We show under what conditions it is possible to obtain a linear atomic chain via pulling of the graphene nanoribbons. The presence of dimers composed of two-coordinated carbon atoms at the edge of the ribbons is necessary for the formation of the linear chains, otherwise there is simply the full rupture of the structure. The presence of Stone-Wales defects close to these dimers may lead to the formation of longer chains. The local atomic configuration of the suspended atoms indicates the formation of single and triple bonds, which is a characteristic of polyynes.

Atomistic molecular dynamics study of interface formation: Al on poly(p-phenylene vinylene)

We model interface formation by metal deposition on the conjugated polymer poly-para-phenylene vinylene, studying direct aluminum and layered aluminum-calcium structures Al/PPV and Al/Ca/PPV. To do that we use classical molecular dynamics simulations, checked by ab initio density-functional theory calculations, for selected relevant configurations. We find that Al not only migrates easily into the film, with a strong charge transfer to the neighboring chains, but also promotes rearrangement of the polymer in the interfacial region to the hexagonal structure. On the other hand, our results indicate that a thin Ca layer is sufficient to protect the film and maintain a well-defined metal/polymer interface, and that also a thin Al capping layer may protect the whole from environmental degradation.

Defect-induced effects on carrier migration through one-dimensional poly(para-phenylenevinylene) chains

Defects in one-dimensional (1D) systems can be intrinsically distinct from its three-dimensional counterparts, and polymer films are good candidates for showing both extremes that are difficult to individuate in the experimental data. We study theoretically the impact of simple hydrogen and oxygen defects on the electron transport properties of one-dimensional poly(para-phenylenevinylene) chains through a multiscale technique, starting from classical structural simulations for crystalline films to extensive ab initio calculations within density functional theory for the defects in single crystalline-constrained chains. The most disruptive effect on carrier transport comes from conjugation breaking imposed by the overcoordination of a carbon atom in the vinyl group independently from the chemical nature of the defect. The particular case of the [C=O] (keto-defect) shows in addition unexpected electron-hole separation, suggesting that the experimentally detected photoluminescence bleaching and photoconductivity enhancement could be due to exciton dissociation caused by the 1D characteristics of the defect.

Modeling the dichroic absorption band edge and light-induced magnetism in EuTe

The band-edge optical absorption in EuTe is studied in the framework of the 5d conduction band atomic model. Both relaxed antiferromagnetic order, and ferromagnetic order induced by an external magnetic field, were analyzed. For ferromagnetic arrangement, the absorption is characterized by a hugely dichroic doublet of narrow lines. In the antiferromagnetic order, the spectrum is blueshifted, becomes much broader and weaker, and dichroism is suppressed. These results are in excellent qualitative and quantitative agreement with experimental observations on EuTe and EuSe published by us previously [Phys. Rev. B 72, 155337 (2005)]. The possibility of inducing ferromagnetic order by illuminating the material at photon energies resonant with the band gap is also discussed.

The conversion of phase to amplitude fluctuations of a light beam by an optical cavity

Very low intensity and phase fluctuations are present in a bright light field such as a laser beam. These subtle quantum fluctuations may be used to encode quantum information. Although intensity is easily measured with common photodetectors, accessing the phase information requires interference experiments. We introduce one such technique, the rotation of the noise ellipse of light, which employs an optical cavity to achieve the conversion of phase to intensity fluctuations. We describe the quantum noise of light and how it can be manipulated by employing an optical resonance technique and compare it to similar techniques, such as Pound - Drever - Hall laser stabilization and homodyne detection. (c) 2008 American Association of Physics Teachers.

Scalable continuous-variable entanglement of light beams produced by optical parametric oscillators

We show that scalable multipartite entanglement among light fields may be generated by optical parametric oscillators (OPOs). The tripartite entanglement existent among the three bright beams produced by a single OPO-pump, signal, and idler-is scalable to a system of many OPOs by pumping them in cascade with the same optical field. This latter serves as an entanglement distributor. The special case of two OPOs is studied, as it is shown that the resulting five bright beams share genuine multipartite entanglement. In addition, the structure of entanglement distribution among the fields can be manipulated to some degree by tuning the incident pump power. The scalability to many fields is straightforward, allowing an alternative implementation of a multipartite quantum information network with continuous variables.

Renyi entropy and parity oscillations of anisotropic spin-s Heisenberg chains in a magnetic field

Using the density matrix renormalization group, we investigate the Renyi entropy of the anisotropic spin-s Heisenberg chains in a z-magnetic field. We considered the half-odd-integer spin-s chains, with s = 1/2, 3/2, and 5/2, and periodic and open boundary conditions. In the case of the spin-1/2 chain we were able to obtain accurate estimates of the new parity exponents p(alpha)((p)) and p(alpha)((o)) that gives the power-law decay of the oscillations of the alpha-Renyi entropy for periodic and open boundary conditions, respectively. We confirm the relations of these exponents with the Luttinger parameter K, as proposed by Calabrese et al. [Phys. Rev. Lett. 104, 095701 (2010)]. Moreover, the predicted periodicity of the oscillating term was also observed for some nonzero values of the magnetization m. We show that for s > 1/2 the amplitudes of the oscillations are quite small and get accurate estimates of p(alpha)((p)) and p(alpha)((o)) become a challenge. Although our estimates of the new universal exponents p(alpha)((p)) and p(alpha)((o)) for the spin-3/2 chain are not so accurate, they are consistent with the theoretical predictions.

Anisotropic Confinement, Electronic Coupling and Strain Induced Effects Detected by Valence-Band Anisotropy in Self-Assembled Quantum Dots

A method to determine the effects of the geometry and lateral ordering on the electronic properties of an array of one-dimensional self-assembled quantum dots is discussed. A model that takes into account the valence-band anisotropic effective masses and strain effects must be used to describe the behavior of the photoluminescence emission, proposed as a clean tool for the characterization of dot anisotropy and/or inter-dot coupling. Under special growth conditions, such as substrate temperature and Arsenic background, 1D chains of In(0.4)Ga(0.6) As quantum dots were grown by molecular beam epitaxy. Grazing-incidence X-ray diffraction measurements directly evidence the strong strain anisotropy due to the formation of quantum dot chains, probed by polarization-resolved low-temperature photoluminescence. The results are in fair good agreement with the proposed model.

Antimicrobial photodynamic action on dentin using a light-emitting diode light source

Objective: The aim of this study was the evaluation of two different photosensitizers activated by red light emitted by light-emitting diodes (LEDs) in the decontamination of carious bovine dentin. Materials and Methods: Fifteen bovine incisors were used to obtain dentin samples which were immersed in brain-heart infusion culture medium supplemented with 1% glucose, 2% sucrose, and 1% young primary culture of Lactobacillus acidophilus 108 CFU/mL and Streptococcus mutans 108 CFU/mL for caries induction. Three different concentrations of the Photogem solution, a hematoporphyrin derivative (1, 2, and 3 mg/mL) and two different concentrations of toluidine blue O (TBO), a basic dye (0.025 and 0.1 mg/mL) were used. To activate the photosensitizers two different light exposure times were used: 60 sec and 120 sec, corresponding respectively to the doses of 24 J/cm(2) and 48 J/cm(2). Results: After counting the numbers of CFU per milligram of carious dentin, we observed that the use of LED energy in association with Photogem or TBO was effective for bacterial reduction in carious dentin, and that the greatest effect on S. mutans and L. acidophilus was obtained with TBO at 0.1 mg/mL and a dose of 48 J/cm(2). It was also observed that the overall toxicity of TBO was higher than that of Photogem, and that the phototoxicity of TBO was higher than that of Photogem. Conclusion: Based on our data we propose a mathematical model for the photodynamic effect when different photosensitizer concentrations and light doses are used.

Ground-state functional for quantum spin chains with bond defects

We have developed a nonlocal functional of the exchange interaction for the ground-state energy of quantum spin chains described by the Heisenberg Hamiltonian. An alternating chain is used to obtain the correlation energy and a local unit-cell approximation is defined in the context of the density-functional theory. The agreement with our exact numerical data, for small chains, is significantly better than a previous formulation, even for chains with several ferromagnetic or antiferromagnetic bond defects. The results can be particularly relevant in the study of finite spin-1/2 Heisenberg chains, with exchange couplings changing, magnitude, or even sign, from bond-to-bond.

The electromagnetics of light transmission through subwavelength slits in metallic films

By numerically calculating the relevant electromagnetic fields and charge current densities, we show how local charges and currents near subwavelength structures govern light transmission through subwavelength apertures in a real metal film. The illumination of a single aperture generates surface waves; and in the case of slits, generates them with high efficiency and with a phase close to - pi with respect to a reference standing wave established at the metal film front facet. This phase shift is due to the direction of induced charge currents running within the slit walls. The surface waves on the entrance facet interfere with the standing wave. This interference controls the profile of the transmission through slit pairs as a function of their separation. We compare the calculated transmission profile for a two-slit array to simple interference models and measurements [Phys. Rev. B 77(11), 115411 (2008)]. (C) 2011 Optical Society of America

Can we make a Bohmian electron reach the speed of light, at least for one instant?

In Bohmian mechanics, a version of quantum mechanics that ascribes world lines to electrons, we can meaningfully ask about an electron's instantaneous speed relative to a given inertial frame. Interestingly, according to the relativistic version of Bohmian mechanics using the Dirac equation, a massive particle's speed is less than or equal to the speed of light, but not necessarily less. That is, there are situations in which the particle actually reaches the speed of light-a very nonclassical behavior. That leads us to the question of whether such situations can be arranged experimentally. We prove a theorem, Theorem 5, implying that for generic initial wave functions the probability that the particle ever reaches the speed of light, even if at only one point in time, is zero. We conclude that the answer to the question is no. Since a trajectory reaches the speed of light whenever the quantum probability current (psi) over bar gamma(mu)psi is a lightlike 4-vector, our analysis concerns the current vector field of a generic wave function and may thus be of interest also independently of Bohmian mechanics. The fact that the current is never spacelike has been used to argue against the possibility of faster-than-light tunneling through a barrier, a somewhat similar question. Theorem 5, as well as a more general version provided by Theorem 6, are also interesting in their own right. They concern a certain property of a function psi : R(4) -> C(4) that is crucial to the question of reaching the speed of light, namely being transverse to a certain submanifold of C(4) along a given compact subset of space-time. While it follows from the known transversality theorem of differential topology that this property is generic among smooth functions psi : R(4) -> C(4), Theorem 5 asserts that it is also generic among smooth solutions of the Dirac equation. (C) 2010 American Institute of Physics. [doi:10.1063/1.3520529]

Knoop Microhardness and FT-Raman Spectroscopic Evaluation of a Resin-Based Dental Material Light-Cured by an Argon Ion Laser and Halogen Lamp: An in Vitro Study

Objective: The purpose of this study was to evaluate in vitro the Knoop microhardness (Knoop hardness number [KHN]) and the degree of conversion using FT-Raman spectroscopy of a light-cured microhybrid resin composite (Z350-3M-ESPE) Vita shade A3 photopolymerized with a halogen lamp or an argon ion laser. Background Data: Optimal polymerization of resin-based dental materials is important for longevity of restorations in dentistry. Materials and Methods: Thirty specimens were prepared and inserted into a disc-shaped polytetrafluoroethylene mold that was 2.0 mm thick and 3 mm in diameter. The specimens were divided into three groups (n = 10 each). Group 1 (G1) was light-cured for 20 sec with an Optilux 501 halogen light with an intensity of 1000 mW/cm(2). Group 2 (G2) was photopolymerized with an argon laser with a power of 150 mW for 10 sec, and group 3 (G3) was photopolymerized with an argon laser at 200 mW of power for 10 sec. All specimens were stored in distilled water for 24 h at 37 degrees C and kept in lightproof containers. For the KHN test five indentations were made and a depth of 100 mu m was maintained in each specimen. One hundred and fifty readings were obtained using a 25-g load for 45 sec. The degree of conversion values were measured by Raman spectroscopy. KHN and degree of conversion values were obtained on opposite sides of the irradiated surface. KHN and degree of conversion data were analyzed by one-way ANOVA and Tukey tests with statistical significance set at p < 0.05. Results: The results of KHN testing were G1 = 37.428 +/- 4.765; G2 = 23.588 +/- 6.269; and G3 = 21.652 +/- 4.393. The calculated degrees of conversion (DC%) were G1 = 48.57 +/- 2.11; G2 = 43.71 +/- 3.93; and G3 = 44.19 +/- 2.71. Conclusions: Polymerization with the halogen lamp ( G1) attained higher microhardness values than polymerization with the argon laser at power levels of 150 and 200 mW; there was no difference in hardness between the two argon laser groups. The results showed no statistically significant different degrees of conversion for the polymerization of composite samples with the two light sources tested.

Structure, stability, depolarized light scattering, and vibrational spectra of fullerenols from all-electron density-functional-theory calculations

We have investigated the stability, electronic properties, Rayleigh (elastic), and Raman (inelastic) depolarization ratios, infrared and Raman absorption vibrational spectra of fullerenols [C(60)(OH)(n)] with different degrees of hydroxylation by using all-electron density-functional-theory (DFT) methods. Stable arrangements of these molecules were found by means of full geometry optimizations using Becke's three-parameter exchange functional with the Lee, Yang, and Parr correlation functional. This DFT level has been combined with the 6-31G(d,p) Gaussian-type basis set, as a compromise between accuracy and capability to treat highly hydroxylated fullerenes, e.g., C(60)(OH)(36). Thus, the molecular properties of fullerenols were systematically analyzed for structures with n=1, 2, 3, 4, 8, 10, 16, 18, 24, 32, and 36. From the electronic structure analysis of these molecules, we have evidenced an important effect related to the weak chemical reactivity of a possible C(60)(OH)(24) isomer. To investigate Raman scattering and the vibrational spectra of the different fullerenols, frequency calculations are carried out within the harmonic approximation. In this case a systematic study is only performed for n=1-4, 8, 10, 16, 18, and 24. Our results give good agreements with the expected changes in the spectral absorptions due to the hydroxylation of fullerenes.