Vortex ring reconnections in elliptic Gaussian beams.

We study theoretically accessible optical vortex rings and their topological transformations in superpositions of elliptic and astigmatic Gaussian beams with a plane wave. We demonstrate the birth and death of vortex rings at isolated points on and off the optical axis and their pairwise and higher-order three- and four-ring reconnections.

Vortex rings in paraxial laser beams.

Interference of a fundamental vortex-free Gaussian beam with a co-propagating plane wave leads to nucleation of a series of vortex rings in the planes transverse to the optical axis; the number of rings grows with vanishing amplitude of the plane wave. In contrast, such interference with a beam carrying on-axis vortex with winding number l results in the formation of |l| rings elongated and gently twisted in propagation direction. The twist handedness of the vortex lines is determined by the interplay between dynamic and geometric phases of the Gaussian beam and the twist angle grows with vanishing amplitude of the plane wave. In the counter-propagating geometry the vortex rings nucleate and twist with half-wavelength period dominated by the interference grating in propagation direction.

Conical Diffraction and Composite Lieb Bosons in Photonic Lattices.

Pseudospin describes how waves are distributed between different "internal" degrees of freedom or microscopic states, such as polarizations, sublattices, or layers. Here, we experimentally demonstrate and explain wave dynamics in a photonic Lieb lattice, which hosts an integer pseudospin s=1 conical intersection. We study the most striking differences displayed by integer pseudospin states: pseudospin-dependent conical diffraction and the generation of higher charged optical vortices.

Changing image of correlation optics: introduction.

This feature issue of Applied Optics contains a series of selected papers reflecting recent progress of correlation optics and illustrating current trends in vector singular optics, internal energy flows at light fields, optical science of materials, and new biomedical applications of lasers.

Creation of orbital angular momentum states with chiral polaritonic lenses.

Controlled transfer of orbital angular momentum to an exciton-polariton Bose-Einstein condensate spontaneously created under incoherent, off resonant excitation conditions is a long-standing challenge in the field of microcavity polaritonics. We demonstrate, experimentally and theoretically, a simple and efficient approach to the generation of nontrivial orbital angular momentum states by using optically induced potentials-chiral polaritonic lenses. These lenses are produced by a structured optical pump with a spatial distribution of intensity that breaks the chiral symmetry of the system.

Dynamics of three-Airy beams carrying optical vortices.

We study numerically and demonstrate experimentally a novel type of singular optical beams formed by the phase imprinting of an optical vortex into the structure of the three-Airy beams. In contrast to a vortex-free product of three Airy beams, in this type of singular-Airy beam, the vortex in the beam axis causes a twist in the beam transverse intensity profile with propagation. Such a new type of singular beams appears especially attractive for applications in optical micromanipulation.

Correlation optics in progress: introduction to the feature issue.

This feature issue of Applied Optics contains a series of selected papers reflecting recent progress of correlation optics and showing, in part, the trend from micro-optics to nano-optics.

Discrete vortex solitons and parity time symmetry.

We study the effect of lifting the degeneracy of vortex modes with a parity time (PT) symmetric defect, using discrete vortices in a circular array of nonlinear waveguides as an example. When the defect is introduced, the degenerate linear vortex modes spontaneously break PT symmetry and acquire complex eigenvalues, but nonlinear propagating modes with real propagation constants can still exist. The stability of nonlinear modes depends on both the magnitude and the sign of the vortex charge; thus PT symmetric systems offer new mechanisms to control discrete vortices.

Self-induced mode transformation in nonlocal nonlinear media.

We report on the first experimental observation of self-induced optical mode transformations in nonlocal nonlinear media. We show that the quadrupole Hermite-Gaussian mode experiences complex nonlinear dynamics in a nematic liquid crystal, including power-dependent conversion into a radially symmetric Laguerre-Gaussian mode. The physical mechanism responsible for self-induced transformation is the excitation of internal modes of a metastable quadrupole nonlocal soliton and its subsequent transmutation into a robust soliton with a bright peak surrounded by a bright ring. We also observe the onset of transformations of higher-order modes, proving the generic character of this nonlinear phenomenon.

Optical vortices at Fano resonances.

We study the transmission properties of a one-dimensional array with intersite side-coupled defect and demonstrate that, as the system undergoes sharp Fano resonance, it exhibits geometry-induced vorticity of the energy flux. The optical vortex inverts its handedness at the Fano resonance, revealing a strong link between two phenomena, both being of interference origin. Our approach is quite general and can be applied to a variety of systems; as an example we demonstrate a similar effect in photonic crystal structure.

Observation of vector solitons with hidden vorticity.

This letter reports the first experimental observation, to our knowledge, of optical vector solitons composed of two incoherently coupled vortex components. We employ nematic liquid crystal to generate stable vector solitons with counterrotating vortices and hidden vorticity. In contrast, the solitons with explicit vorticity and corotating vortex components show azimuthal splitting.

Knotted solitons in nonlinear magnetic metamaterials.

We demonstrate that nonlinear magnetic metamaterials comprised of a lattice of weakly coupled split-ring resonators driven by an external electromagnetic field may support entirely new classes of spatially localized modes--knotted solitons, which are stable self-localized dissipative structures in the form of closed knotted chains. We demonstrate different topological types of stable knots for the subcritical coupling between resonators and instability-induced breaking of the chains for the supercritical coupling.

Dipole azimuthons and vortex charge flipping in nematic liquid crystals.

We demonstrate self-trapped laser beams carrying phase singularities in nematic liquid crystals. We experimentally observe the astigmatic transformation of vortex beams into spiraling dipole azimuthons accompanied by power-dependent charge-flipping of the on-axis phase singularity. The latter topological reactions involve triplets of vortex lines and resemble pitchfork bifurcations.

Discrete multivortex solitons.

We introduce discrete multivortex solitons in a ring of nonlinear oscillators coupled to a central site. Regular clusters of discrete vortices appear as a result of mode collisions, and we show that their stability is determined by global symmetries rather than the stability of constituent vortices. Stable multivortex solitons support complex vortex dynamics including charge flipping and spiraling.

Multimode nematicon waveguides.

We report on the first (to our knowledge) experimental observation of higher-order modes guided by soliton-induced waveguides in nematic liquid crystals. We find that the nematicon waveguides operate in a bounded power region specific to each guided mode. Below this region, the guided beams diffract; above this region, the mode mixing and coupling give rise to an unstable output.

Dynamic diffraction and interband transitions in two-dimensional photonic lattices.

We reveal a direct link between two fundamental wave phenomena in periodic media, Pendellösung oscillations and resonant coupling between spectral bands. We experimentally measure the power transfer between laser beams associated with the high-symmetry points in periodic and biased hexagonal photonic lattices. As a result, we demonstrate that Pendellösung oscillations dominate the dynamics of resonant interband transitions on a short propagation scale.

Counterpropagating nematicons in bias-free liquid crystals.

We experimentally investigate the interaction of two counter-propagating spatial optical solitons (nematicons) in bias-free nematic liquid crystals. We demonstrate the existence of vector solitons composed of two nematicons propagating in opposite directions and analyze their stability versus relative distance and input power.We observe the dynamic instability of two counterpropagating nematicons in the form of time-dependent splitting and spatial entanglement.

Selective trapping of multiple particles by volume speckle field.

We suggest a new approach for selective trapping of light absorbing particles in gases by multiple optical bottle-beam-like traps created by volume speckle field. We demonstrate stable simultaneous confinement of a few thousand micro-particles in air with a single lowpower laser beam. The size distribution of trapped particles exhibits a narrow peak near the average size of an optical speckle. Thus, the speckleformed traps act as a sieve with the holes selecting particles of a similar size.

Spatially engineered polarization states and optical vortices in uniaxial crystals.

We describe how the propagation of light through uniaxial crystals can be used as a versatile tool towards the spatial engineering of polarization and phase, thereby providing an all-optical technique for vectorial and scalar singular beam shaping in optics. Besides the prominent role played by the linear birefringence, the influence of circular birefringence (the optical activity) is discussed as well and both the monochromatic and polychromatic singular beam shaping strategies are addressed. Under cylindrically symmetric light-matter interaction, the radially, azimuthally, and spirally polarized eigen-modes for the light field are revealed to be of a fundamental interest to describe the physical mechanisms at work when dealing with scalar and vectorial optical singularities. In addition, we also report on nontrivial effects arising from cylindrical symmetry breaking, e.g. tilting the incident beam with respect to the crystal optical axis.

Soliton bending and routing induced by interaction with curved surfaces in nematic liquid crystals.

We study experimentally the interaction of spatial optical solitons with curved dielectric surfaces in unbiased nematic liquid crystals. We demonstrate that this interaction depends on the curvature of the surface and the walk-off, and it can be employed for efficient routing and control of the soliton trajectories. We also observe a large-angle total internal reflection of the soliton beam from an interface between liquid crystal and air.