Stable three-dimensional spinning optical solitons supported by competing quadratic and cubic nonlinearities

We show that the quadratic interaction of fundamental and second harmonics in a bulk dispersive medium, combined with self-defocusing cubic nonlinearity, give rise to completely localized spatiotemporal solitons (vortex tori) with vorticity s=1. There is no threshold necessary for the existence of these solitons. They are found to be stable against small perturbations if their energy exceeds a certain critical value, so that the stability domain occupies about 10% of the existence region of the solitons. We also demonstrate that the s=1 solitons are stable against very strong perturbations initially added to them. However, on the contrary to spatial vortex solitons in the same model, the spatiotemporal solitons with s=2 are never stable.Comment: latex text, 10 ps and 2 jpg figures; Physical Review E, in pres

Nonlinearity Management in Higher Dimensions

In the present short communication, we revisit nonlinearity management of the time-periodic nonlinear Schrodinger equation and the related averaging procedure. We prove that the averaged nonlinear Schrodinger equation does not support the blow-up of solutions in higher dimensions, independently of the strength in the nonlinearity coefficient variance. This conclusion agrees with earlier works in the case of strong nonlinearity management but contradicts those in the case of weak nonlinearity management. The apparent discrepancy is explained by the divergence of the averaging procedure in the limit of weak nonlinearity management.Comment: 9 pages, 1 figure

Polychromatic solitons in a quadratic medium

We introduce the simplest model to describe parametric interactions in a quadratically nonlinear optical medium with the fundamental harmonic containing two components with (slightly) different carrier frequencies [which is a direct analog of wavelength-division multiplexed (WDM) models, well known in media with cubic nonlinearity]. The model takes a closed form with three different second-harmonic components, and it is formulated in the spatial domain. We demonstrate that the model supports both polychromatic solitons (PCSs), with all the components present in them, and two types of mutually orthogonal simple solitons, both types being stable in a broad parametric region. An essential peculiarity of PCS is that its power is much smaller than that of a simple (usual) soliton (taken at the same values of control parameters), which may be an advantage for experimental generation of PCSs. Collisions between the orthogonal simple solitons are simulated in detail, leading to the conclusion that the collisions are strongly inelastic, converting the simple solitons into polychromatic ones, and generating one or two additional PCSs. A collision velocity at which the inelastic effects are strongest is identified, and it is demonstrated that the collision may be used as a basis to design a simple all-optical XOR logic gate.Comment: 9 pages, 8 figures, accepted to Phys. Rev.

Observation of spontaneous self-channelling of light in air below the collapse threshold

We report the observation of the self-guided propagation of 120 fs, 0.56 mJ infrared radiation in air for distances greater than one meter. In contrast to the known case of filamentation, in the present experiment the laser power is lower than the collapse threshold. Therefore the counter balance between Kerr self focussing and ionization induced defocussing as the stabilizing mechanism is ruled out. Instead, we find evidences of a process in which the transversal beam distribution reshapes into a form similar to a Townes soliton. We include numerical support for this conclusion

Rotating optical soliton clusters

We introduce the concept of soliton clusters -- multi-soliton bound states in a homogeneous bulk optical medium, and reveal a key physical mechanism for their stabilization associated with a staircase-like phase distribution that induces a net angular momentum and leads to cluster rotation. The ringlike soliton clusters provide a nontrivial generalization of the concepts of two-soliton spiraling, optical vortex solitons, and necklace-type optical beams.Comment: 4 pages, 5 figure

Stable spinning optical solitons in three dimensions

We introduce spatiotemporal spinning solitons (vortex tori) of the three-dimensional nonlinear Schrodinger equation with focusing cubic and defocusing quintic nonlinearities. The first ever found completely stable spatiotemporal vortex solitons are demonstrated. A general conclusion is that stable spinning solitons are possible as a result of competition between focusing and defocusing nonlinearities.Comment: 4 pages, 6 figures, accepted to Phys. Rev. Let

Target, audit and risk assessment cultures in the probation service

This article traces the rise of managerialism in the probation service in England and Wales before exploring the impact of these changes through reference to in-depth observation and interviews in probation. The article considers how national standards affect practice; how audits feature and their impact on accountability; and how the use of risk assessment tools are perceived and resisted in two probation teams in England Wales. The article then turns to changes implemented by the Coalition Government and highlights some tensions between managerialist occupational cultures in probation and what might occur in the near future

Search for right-handed W bosons in top quark decay

We present a measurement of the fraction f+ of right-handed W bosons produced in top quark decays, based on a candidate sample of $t\bar{t}$ events in the lepton+jets decay mode. These data correspond to an integrated luminosity of 230pb^-1, collected by the DO detector at the Fermilab Tevatron $p\bar{p}$ Collider at sqrt(s)=1.96 TeV. We use a constrained fit to reconstruct the kinematics of the $t\bar{t}$ and decay products, which allows for the measurement of the leptonic decay angle $\theta^*$ for each event. By comparing the $\cos\theta^*$ distribution from the data with those for the expected background and signal for various values of f+, we find f+=0.00+-0.13(stat)+-0.07(syst). This measurement is consistent with the standard model prediction of f+=3.6x10^-4.Comment: Submitted to Physical Review D Rapid Communications 7 pages, 3 figure

Multivariate searches for single top quark production with the D0 detector

We present a search for electroweak production of single top quarks in the s-channel (p-pbar -> t bbar + X) and t-channel (p-pbar -> tq bbar + X) modes. We have analyzed 230 pb^(-1) of data collected with the D0 detector at the Fermilab Tevatron collider at a center-of-mass energy of sqrt(s) = 1.96 TeV. Two separate analysis methods are used: neural networks and a cut-based analysis. No evidence for a single top quark signal is found. We set 95% confidence level upper limits on the production cross sections using Bayesian statistics, based on event counts and binned likelihoods formed from the neural network output. The limits from the neural network (cut-based) analysis are 6.4 pb (10.6 pb) in the s-channel and 5.0 pb (11.3 pb) in the t-channel.Comment: submitted to Phys. Rev. D, Fermilab preprint Fermilab-Pub-06/069-