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Singularities, i.e. places of discontinuities of parameters are extremely general objects appearing in electromagnetic waves and thus are the key to understanding fundamental wave processes. These structures commonly occur in purely coherent, highly directional waves, such as laser beams, determining additional spatial or "`topological"' properties of the wave fields independently of their propagational dynamics. For instance, topologies of wave fronts, called phase singularities, add orbital degrees of freedom to electromagnetic waves. These singularities are common to all types of scalar waves described only by their intensity and phase distributions. As the electromagnetic wave is a vector wave, its topological properties generally depend on all field components leading to complex field patterns in space and time. These patterns may contain singular points of undefined instantaneous orientation of the vector field, i.e. the instantaneous field (IF) singularities. In zero-order paraxial approximation of purely transverse electromagnetic waves, some instantaneous field distributions may carry apparent topological monopoles, when the originally source-free wave exhibits spatial structure associated with the 2D "virtual" sources of electromagnetic fields. Here we present systematic description of topological singularities in both electric and magnetic field of the electromagnetic waves in a paraxial approximation. We also consider the important types of paraxial electromagnetic waves with complex transverse field structures containing instantaneous field singularities., Comment: 13 pages, 6 figures (revised)

We study the propagation of light beams through optical media with competing nonlocal nonlinearities. We demonstrate that the nonlocality of competing focusing and defocusing nonlinearities gives rise to self-organization and stationary states with stable hexagonal intensity patterns, akin to transverse crystals of light filaments. Signatures of this long-range ordering are shown to be observable in the propagation of light in optical waveguides and even in free space. We consider a specific form of the nonlinear response that arises in atomic vapor upon proper light coupling. Yet, the general phenomenon of self-organization is a generic consequence of competing nonlocal nonlinearities, and may, hence, also be observed in other settings., Comment: 4 pages, 4 figures

We employ conical refraction of light in a biaxial crystal to create an optical bottle for trapping and manipulation of particles. We show that by just varying the polarization of the input light the bottle can be opened and closed at will. We experimentally demonstrate stable photophoretic trapping and controllable loading and unloading of light absorbing particles in the trap., Comment: 4 pages, 5 figures

Quasiperiodic oscillations and shape-transformations of higher-order bright solitons in nonlinear nonlocal media have been frequently observed in recent years, however, the origin of these phenomena was never completely elucidated. In this paper, we perform a linear stability analysis of these higher-order solitons by solving the Bogoliubov-de Gennes equations. This enables us to understand the emergence of a new oscillatory state as a growing unstable mode of a higher-order soliton. Using dynamically important states as a basis, we provide low-dimensional visualizations of the dynamics and identify quasiperiodic and homoclinic orbits, linking the latter to shape-transformations.

We consider the interplay between nonlocal nonlinearity and randomness for two different nonlinear Schr\"odinger models. We show that stability of bright solitons in presence of random perturbations increases dramatically with the nonlocality-induced finite correlation length of the noise in the transverse plane, by means of both numerical simulations and analytical estimates. In fact, solitons are practically insensitive to noise when the correlation length of the noise becomes comparable to the extent of the wave packet. We characterize soliton stability using two different criteria based on the evolution of the Hamiltonian of the soliton and its power. The first criterion allows us to estimate a time (or distance) over which the soliton preserves its form. The second criterion gives the life-time of the solitary wave packet in terms of its radiative power losses. We derive a simplified mean field approach which allows us to calculate the power loss analytically in the physically relevant case of weakly correlated noise, which in turn serves as a lower estimate of the life-time for correlated noise in general case.

We propose a scheme for the creation of stable three dimensional bright solitons in Bose-Einstein condensates, i.e., the matter-wave analog of so-called spatio-temporal "light bullets". Off-resonant dressing to Rydberg $nD$-states is shown to provide nonlocal attractive interactions, leading to self-trapping of mesoscopic atomic clouds by a collective excitation of a Rydberg atom pair. We present detailed potential calculations, and demonstrate the existence of stable solitons under realistic experimental conditions by means of numerical simulations., Comment: 4 pages, 5 figures

We discuss spatial dynamics and collapse scenarios of localized waves governed by the nonlinear Schr\"{o}dinger equation with nonlocal nonlinearity. Firstly, we prove that for arbitrary nonsingular attractive nonlocal nonlinear interaction in arbitrary dimension collapse does not occur. Then we study in detail the effect of singular nonlocal kernels in arbitrary dimension using both, Lyapunoff's method and virial identities. We find that for for a one-dimensional case, i.e. for $n=1$, collapse cannot happen for nonlocal nonlinearity. On the other hand, for spatial dimension $n\geq2$ and singular kernel $\sim 1/r^\alpha$, no collapse takes place if $\alpha<2$, whereas collapse is possible if $\alpha\ge2$. Self-similar solutions allow us to find an expression for the critical distance (or time) at which collapse should occur in the particular case of $\sim 1/r^2$ kernels. Moreover, different evolution scenarios for the three dimensional physically relevant case of Bose Einstein condensate are studied numerically for both, the ground state and a higher order toroidal state with and without an additional local repulsive nonlinear interaction. In particular, we show that presence of an additional local repulsive term can prevent collapse in those cases.

We investigate theoretically the interaction of dark solitons in materials with a spatially nonlocal nonlinearity. In particular we do this analytically and for arbitrary degree of nonlocality. We employ the variational technique to show that nonlocality induces an attractive force in the otherwise repulsive soliton interaction., Comment: submitted for publication

We investigate properties of dark solitons in nonlocal materials with an arbitrary degree of nonlocality. We employ the variational technique and describe the dark solitons, for the first time, in the whole range of degree of nonlocality., Comment: to be published in Optics Letters

We study formation of rotating three-dimensional high-order solitons (azimuthons) in Bose Einstein condensate with attractive nonlocal nonlinear interaction. In particular, we demonstrate formation of toroidal rotating solitons and investigate their stability. We show that variational methods allow a very good approximation of such solutions and predict accurately the soliton rotation frequency. We also find that these rotating localized structures are very robust and persist even if the initial condensate conditions are rather far from the exact soliton solutions. Furthermore, the presence of repulsive contact interaction does not prevent the existence of those solutions, but allows to control their rotation. We conjecture that self-trapped azimuthons are generic for condensates with attractive nonlocal interaction.

We study the formation of azimuthons, i.e., rotating spatial solitons, in media with nonlocal focusing nonlinearity. We show that whole families of these solutions can be found by considering internal modes of classical non-rotating stationary solutions, namely vortex solitons. This offers an exhaustive method to identify azimuthons in a given nonlocal medium. We demonstrate formation of azimuthons of different vorticities and explain their properties by considering the strongly nonlocal limit of accessible solitons., Comment: 11 pages, 7 figures

We discuss generic properties of rotating nonlinear wave solutions, the so called azimuthons, in nonlocal media. Variational methods allow us to derive approximative values for the rotating frequency, which is shown to depend crucially on the nonlocal response function. Further on, we link families of azimuthons to internal modes of classical non-rotating stationary solutions, namely vortex and multipole solitons. This offers an exhaustive method to identify azimuthons in a given nonlocal medium., Comment: 14 pages, 9 figures, 3 movies (external links)

We study cascaded quadratic soliton compressors and address the physical mechanisms that limit the compression. A nonlocal model is derived, and the nonlocal response is shown to have an additional oscillatory component in the nonstationary regime when the group-velocity mismatch (GVM) is strong. This inhibits efficient compression. Raman-like perturbations from the cascaded nonlinearity, competing cubic nonlinearities, higher-order dispersion, and soliton energy may also limit compression, and through realistic numerical simulations we point out when each factor becomes important. We find that it is theoretically possible to reach the single-cycle regime by compressing high-energy fs pulses for wavelengths $\lambda=1.0-1.3 \mu{\rm m}$ in a $\beta$-barium-borate crystal, and it requires that the system is in the stationary regime, where the phase mismatch is large enough to overcome the detrimental GVM effects. However, the simulations show that reaching single-cycle duration is ultimately inhibited by competing cubic nonlinearities as well as dispersive waves, that only show up when taking higher-order dispersion into account., Comment: 16 pages, 5 figures, submitted to Optics Express

In this paper we analyze the existence, stability, dynamical formation and mobility properties of localized solutions in a one-dimensional system described by the discrete nonlinear Schr\"{o}dinger equation with a linear point defect. We consider both attractive and repulsive defects in a focusing lattice. Among our main findings are: a) the destabilization of the on--site mode centered at the defect in the repulsive case; b) the disappearance of localized modes in the vicinity of the defect due to saddle-node bifurcations for sufficiently strong defects of either type; c) the decrease of the amplitude formation threshold for attractive and its increase for repulsive defects; and d) the detailed elucidation as a function of initial speed and defect strength of the different regimes (trapping, trapping and reflection, pure reflection and pure transmission) of interaction of a moving localized mode with the defect., Comment: 12 pages, 10 figures

We show that ballistic transport of optically excited atoms in an atomic vapor provides a nonlocal nonlinearity which stabilizes the propagation of vortex beams and higher order modes in the presence of a self-focusing nonlinearity. Numerical experiments demonstrate stable propagation of lowest and higher order vortices over a hundred diffraction lengths, before dissipation leads to decay of these structures., Comment: 3 figures

We discuss existence and stability of two-dimensional solitons in media with spatially nonlocal nonlinear response. We show that such systems, which include thermal nonlinearity and dipolar Bose Einstein condensates, may support a variety of stationary localized structures - including rotating spatial solitons. We also demonstrate that the stability of these structures critically depends on the spatial profile of the nonlocal response function., Comment: 8 pages, 9 figures

We present an overview of recent advances in the understanding of optical beams in nonlinear media with a spatially nonlocal nonlinear response. We discuss the impact of nonlocality on the modulational instability of plane waves, the collapse of finite-size beams, and the formation and interaction of spatial solitons., Comment: Review article, will be published in Journal of Optics B, special issue on Optical Solitons, 6 figures

In the framework of seesaw mechanism with three neutrino flavors, we propose tentatively an efficient parametrization for the spectra of Dirac and righthanded Majorana neutrino mass matrices in terms of three free parameters. Two of them are related to (and determined by) the corresponding parameters introduced previously for the mass spectra of charged leptons and up and down quarks. The third is determined from the experimental estimate of solar $\Delta m^2_{21}$. Then, the atmospheric $\Delta m^2_{32}$ is {\it predicted} close to its experimental estimation. With the use of these three parameters all light active-neutrino masses $ m_1 < m_2 < m_3$ and heavy sterile-neutrino masses $ M_1 < M_2 < M_3$ are readily evaluated. The latter turn out much more {\it hierarchical} than the former. The lightest heavy mass $ M_1$ comes out to be of the order $O(10^6 {\rm GeV})$ so, it is too light to imply that the mechanism of baryogenesis through thermal leptogenesis might work., Comment: 9 pages, no figures

We demonstrate generation and frequency doubling of unit charge vortices in a linear astigmatic resonator. Topological instability of the double charge harmonic vortices leads to well separated vortex cores that are shown to rotate, and become anisotropic, as the resonator is tuned across resonance.

We observe that the {\it invariance} of neutrino mixing matrix under the simultaneous discrete transformations $\nu_1, \nu_2, \nu_3 \to -\nu_1, -\nu_2, \nu_3 $ and $\nu_e, \nu_\mu, \nu_\tau \to -\nu_e, \nu_\tau, \nu_\mu $ (neutrino "horizontal conjugation") {\it characterizes} (as a sufficient condition for it) the familiar bilarge form of neutrino mixing matrix, favored experimentally at present. Thus, the mass neutrinos $\nu_1, \nu_2, \nu_3 $ get a new quantum number, {\it covariant} with respect to their mixings into the flavor neutrinos $\nu_e, \nu_\mu, \nu_\tau $ (neutrino "horizontal parity" equal to -1, -1,1, respectively). The "horizontal parity" turns out to be embedded in a group structure consisting of some Hermitian and real $3\times 3$ matrices $\mu_1, \mu_2, \mu_3 $ and $\phi_1, \phi_2, \phi_3 $, forming pairs interconnected through neutrino mixings. They generate some discrete transformations of mass and flavor neutrinos, respectively, in such a way that the group relations $\mu_1 \mu_2 = \mu_3 $ (cyclic) and $\phi_1 \phi_2 = \phi_3 $ (cyclic) hold, while $\mu_a \mu_b = \mu_b \mu_a $ and $\phi_a \phi_b = \phi_b \phi_a $. Then, for instance, the $\mu_3$ matrix may be chosen equal to the "horizontal parity"., Comment: 1+9 pages, latex, no figures

In the first part of the note, we consider a neutrino texture, where the Dirac and righthanded Majorana masses are proportional. If the former are approximately proportional also to the charged lepton masses, then taking $\Delta m^2_{32}\sim3\times 10^{-3}$ eV$^2$ we estimate approximately that $\Delta m^2_{21}\sim{\cal O}(10^{-5}$ eV$^2)$, what is not very different from the recent KamLAND estimation $\Delta m^2_{21}\sim7\times 10^{-5}$ eV$^2$, consistent with the LMA solar solution. In the second part, we show generically that the {\it invariance} of neutrino mixing matrix under the simultaneous discrete transformations $\nu_e\to-\nu_e$, $\nu_\mu\to\nu_\tau$, $\nu_\tau\to\nu_\mu$ and $\nu_1\to-\nu_1$, $\nu_2\to-\nu_2$, $\nu_3\to\nu_3$ (neutrino "horizontal conjugation") {\it characterizes} the familiar bilarge form of mixing matrix, favored phenomenologically at present. Then, in the case of this form, the mass neutrinos $\nu_1$, $\nu_2$, $\nu_3$ get a new quantum number, {\it covariant} in their mixings (neutrino "horizontal parity", equal to $-1,-1, 1$, respectively). Conversely, such a covariance may be the {\it origin} of the bilarge mixing matrix. In Section 5, the "horizontal parity" is embedded in a group structure., Comment: 21 pages, no figures. Edition of a formula on p. 16 corrected

The author's idea of {\it algebraic compositeness} of fundamental particles, allowing to understand the existence in Nature of three fermion generations, is revisited. It is based on two postulates. i) For all fundamental particles of matter the Dirac square-root procedure $\sqrt{p^2}\to\Gamma^{(N)}\cdot p$ works, leading to a sequence $N=1,2,3,...$ of Dirac-type equations, where four Dirac-type matrices $\Gamma^{(N)}_\mu$ are embedded into a Clifford algebra {\it via} a Jacobi definition introducing four "centre-of-mass" and $(N-1)\times$four "relative" Dirac-type matrices. These define one "centre-of-mass" and $N-1$ "relative" Dirac bispinor indices. ii) The "centre-of-mass" Dirac bispinor index is coupled to the Standard Model gauge fields, while $N-1$ "relative" Dirac bispinor indices are all free undistinguishable physical objects obeying Fermi statistics with the Pauli principle which requires the full antisymmetry with respect to "relative" indices. This allows only for {\it three} $N = 1,3,5$ in the case of $N$ odd, implying the existence of {\it three and only three} generations of fundamental fermions., Comment: 11 pages, no figures

Although the hypothetic sterile neutrino $\nu_s$ is probably not involved significantly in the deficits of solar $\nu_e$'s and atmospheric $\nu_\mu$'s, it may cause the possible LSND effect. In fact, we face such a situation, when the popular nearly bimaximal texture of active neutrinos $\nu_e$, $\nu_\mu$, $\nu_\tau$ is perturbed through a small rotation in the 14 plane, where $\nu_4$ is the extra neutrino mass state induced by the sterile neutrino $\nu_s$. Then, with $m^2_1\simeq m^2_2$ we predict in the simplest case of $s_{13}\to0$ that $\sin^22\theta_{\rm LSND}=s^4_{14}/2$ and $\Delta m^2_{\rm LSND} = |\Delta m^2_{41}|$. However, the negative CHOOZ experiment imposes on $s^4_{14}/2$ the upper bound $1.3\times10^{-3}$, suggesting a reduction of the amplitude of possible LSND effect., Comment: 8 pages, no figures

The conjecture that among three massive neutrinos $\nu_1, \nu_2, \nu_3$ there is no direct mixing between $\nu_1$ and $\nu_3$ leads to a two-mixing texture for three active neutrinos $\nu_e, \nu_\mu, \nu_\tau$. This texture, much discussed previously, is neatly consistent with the observed deficits of solar $\nu_e$'s and atmospheric $\nu_\mu$'s, but (without extra mixing with at least one sterile neutrino $\nu_s$) predicts no LSND effect for accelerator $\nu_\mu$'s. In this option, the masses $m^2_1\stackrel{<}{\sim} m^2_2 << m^2_3$ are readily estimated. The characteristic feature of the two-mixing neutrino texture that {\it only the close neighbours in the hierarchy of massive neutrinos $\nu_1, \nu_2, \nu_3$ mix significantly} may be physically meaningful. Going out from the notion of mixing matrix we construct an intrinsic occupation-number operator whose {\it eigenvalues} 0, 1, 2 {\it numerate the three generations of massive neutrinos}. Analogical constructions work also for charged leptons as well as for up and down quarks., Comment: 9 pages, no figures

We study the scattering properties of optical dipole-mode vector solitons recently predicted theoretically and generated in a laboratory. We demonstrate that such a radially asymmetric composite self-trapped state resembles ``a molecule of light'' which is extremely robust, survives a wide range of collisions, and displays new phenomena such as the transformation of a linear momentum into an angular momentum, etc.We present also experimental verifications of some of our predictions., Comment: 4 pages, 5 postscript figures

A new lefthanded see-saw mechanism is constructed, implying both the smallness of active-neutrino masses and decoupling of heavy passive neutrinos, similarly to the situation in the case of conventional see-saw. But now, in place of the conventional righthanded neutrinos, the lefthanded sterile neutrinos play the role of heavy passive neutrinos, the righthanded neutrinos and righthanded sterile neutrinos being absent. Here, the lefthanded sterile neutrinos are different from charge conjugates of conventional righthanded neutrinos because their lepton numbers differ. In this case, the neutrino mass term is necessarily of pure Majorana type., Comment: LaTeX, 7 pages, no figures, four wrapping errors removed

We derive the renormalization group equations for the neutrino masses and mixing angles in explicit form and discuss the possible classes of their solutions. We identify fixed points in the equations for mixing angles, which can be reached during the evolution for several mass patterns and give $\sin^22\theta_{sol}=\sin^22\theta_{atm}\sin^2\theta_3/ (\sin^2\theta_{atm}\cos^2\theta_3 + \sin^2\theta_3)^2$, consistently with the present experimental information. Further experimental test of this relation is of crucial interest. Moreover, we discuss the stability of quantum corrections to neutrino mass squared differences. Several interesting mass patterns show stability in the presence of fixed point solutions for the angles., Comment: 17 pages, 2 figures. References added and comments concerning the two-flavour case changed

Four-wave mixing and walk-off between two optical beams are investigated for fo cusing Kerr media. It is shown that four-wave mixing reinforces the self-focusi ng of mutually-trapped waves by lowering their power threshold for collapse, on ly when their phase mismatch is small. On the contrary, walk-off inhibits the c ollapse by detrapping the beams, whose partial centroids experience nonlinear o scillations., Comment: 4 pages, 3 figures

The well-known formal analogy between time and absolute temperature, existing on the quantum level, is considered as a profound duality relationship requiring some modifications in the conventional quantum dynamics. They consist of tiny deviations from uniform time run in the physical spacetime, as well as of tiny deviations from unitary time evolution characteristic for the conventional quantum theory. The first deviations are conjectured to be produced by local changes of total average particle number. Then, they imply the second deviations exerting in turn influence upon this particle number. Two examples of the second deviations are described: hypothetic tiny violation of optical theorem for particle scattering, and hypothetic slow variation of the average number of probe particles contained in a sample situated in proximity of a big accelerator (producing abundantly particles on a target)., Comment: 15 pages, LaTeX, no figures

A unified form of mass matrix proposed previously for all fundamental fermions is extended to include the sterile neutrino $\nu_s$, presumed to mix mainly with $\nu_e$, while $\nu_\tau$ mixes strongly with $\nu_\mu$ (and both $\nu_\mu$ and $\nu_\tau$ weakly with $\nu_e$ and $\nu_s$). It turns out that the former mixing can be responsible for oscillations of solar neutrinos, while the latter for those of atmospheric neutrinos. It is mentioned in a footnote that $\nu_s$ is one of two sterile neutrinos, possible from a viewpoint of an algebraic construction. The charged-lepton factor $U^{(e)}$ in the leptonic four-dimensional CKM matrix $V=U^{(\nu) \dagger}U^{(e)}$ is determined through the small deviation of our previous prediction $m_\tau=1776.80$ MeV (valid at the level of $ U^{(e)} = {\bf 1}$) from the experimental value $m_\tau=1777.00^{+0.30}_{-0.27}$ MeV. Then, it gives corrections to the four-dimensional mixing of $\nu_e$, $\nu_\mu$, $\nu_\tau$, $\nu_s$ leading, in particular, to small $\nu_e\to\nu_\mu$, $\nu_e\to \nu_\tau$ and $\nu_\mu\to\nu_s$ oscillations (absent at the level of $U^{(e)}= 1$). However, the LSND estimates of $\bar{\nu}_\mu \to \bar{\nu}_e$ and $\nu_\mu\to\nu_e$ oscillations are too large to be explained by these corrections, though in the case of $\nu_\mu \to\nu_e$ they seem to be at the edge of such possible explanation., Comment: LaTeX, 20 pages, no figures

A unified form of mass matrix is proposed for neutrinos, charged leptons, up quarks and down quarks. Some constraints for the parameters involved are tent% atively postulated. Then, the predictions are neatly consistent with available experimental data. Among the predictions are: (i) $ m_\tau \simeq 1776.80 $~MeV (with the inputs of $ m_e $ and $ m_\mu $), (ii) $ m_{\nu_0} \ll m_{\nu_1} \sim (0.6 to 4)\times 10^{-2} $eV and $ m_{\nu_2} \sim (0.2 to 1)\times 10^{-1} $eV (with the atmospheric-neutrino inpu% ts of $ |m_{\nu_2}^2 - m_{\nu_1}^2| \sim (0.0003 to 0.01) eV ^2 $ and the $\nu_\mu \to \nu_\tau $ oscillation amplitude $\sim 0.8 $), and also (iii) $ m_s \simeq 270 MeV$, $|V_{ub}/V_{cb}| \simeq 0.082 $ and arg$V_{ub} \simeq -64^\circ $ (with the inputs of $ m_c = 1.3 $ GeV, $ m_b = 4.5 $ GeV, $|V_{us}| = 0.221 $ and $|V_{cb}| = 0.041 $, where $ m_u \ll m_c \ll m_t $ and $ m_d \ll m_s \ll m_b $). All elements of the \CKM matrix are evaluated. All elements of its lepton counterpart are calculated up to an unknown phase (Appendix B). Some items related to dynamical aspects of the proposed fermion "texture" are briefly commented on (Appendix A). In particular, the notion of a novel dark matter, free of any Standard--% Model interactions (and their supersymmetric variants), appears in the case of preon option., Comment: 29 pages, LaTeX, no figures