The propagation of a light beam through a nonlinear medium is the simplest scenario that one could imagine for light-matter interaction, but it is accompanied by a series of dramatic and fascinating changes in the spatio-temporal structure of the beam. Among these are light-induced scattering which may result in asymmetric beam fanning; the formation of spatial distributions of the electromagnetic fields that results in back reflection; phase-conjugation; and the possibility to prevent the beam from spreading and to enable selftrapping of the beam profile. Therefore, the spatial evolution of a single light beam is a central topic of nonlinear optical dynamics. A theoretical interpretation of this complex phenomenon and its experimental verification are of key importance for gaining insight into the nonlinear properties of a particular medium, on the one hand, and for contributing to our understanding of more general questions about the complex spatio-temporal behavior of nonlinear systems on the other hand. This is the reason why we have chosen this simple propagation of light through a nonlinear medium as the starting point for our investigations.The topic is not a completely new one. Initial investigations on the selftrapping of light beams in nonlinear media were already performed in the early 1960s [1, 2]. The most important task, since the beginning of this research, was to find the conditions for the stability and nonlinear evolution of solitary-wave solutions of the nonlinear propagation equations associated with the propagation of the beam in the nonlinear material. This is one of the most crucial parts of the problem of self-trapping of optical beams and promises applications in waveguiding, information processing and, most simply, propagation without spreading losses.In general, these effects may occur in all materials that display a nonlinear response that may act in such a way that diffraction effects due to the propagation of the beam can be compensated exactly. The balance between beam broadening on the one hand and self-focusing on the other hand gives rise to the formation of a solitary wave. In this case the envelope of the light wave does not change its profile during propagation. This wave is often called a soliton for short.Solitons are waves that do not spread or disperse like all familiar waves, but retain their size and shape indefinitely -they are dynamically and struc-

We report on the first experimental observation of a novel type of optical vector soliton, a dipole-mode soliton, recently predicted theoretically. We show that these vector solitons can be generated in a photorefractive medium employing two different processes: a phase imprinting, and a symmetry-breaking instability of a vortex-mode vector soliton. The experimental results display remarkable agreement with the theory, and confirm the robust nature of these radially asymmetric two-component solitary waves.Optical spatial solitons in (2+1) dimensions are particle-like solitary waves propagating in a nonlinear bulk medium [1]. The exhaustive research of the past decade has shown that these "light particles" can possess topological phase properties analogous to a charge. Moreover, several light beams can combine to produce a vector soliton with a complex internal structure. This process can be thought of as the formation of a "solitonic molecule" from the constitutuents of different charge.Recently, the existence of the most robust "solitonic molecule", the dipole-mode vector soliton, has been predicted [2]. This novel optical vector soliton originates from trapping of a dipole-mode beam by a waveguide created by a fundamental soliton in the co-propagating, incoherently coupled, beam. While many other topologically complex structures may be created, it is only the dipole mode that is expected to generate a family of dynamically robust vector solitons [2]. The closest counterexample is the vortex-mode vector soliton [3] which has a node-less shape in one component and a ring-like vortex in the other component. This radially symmetric, vector soliton undergoes a nontrivial symmetry-breaking instability [2,4], which transforms it into a more stable object -radially asymmetric dipole-mode vector soliton, even in an isotropic nonlinear medium.While the existence and robustness of the dipole-mode vector solitons have been established theoretically for a general model of an isotropic medium with saturable nonlinearity [2], the main question still stands: Is the stability of these asymmetric solitons, as opposed to the radially symmetric vortex-mode solitons, a fundamental phenomenon that can be demonstrated experimentally?In this Letter we answer this question positively. We observe dipole-mode solitons in strontium barium niobate (SBN) photorefractive crystals experimentally, by employing two different techniques. First, we use a specially fabricated phase mask to create a dipole-like structure in one of the co-propagating, mutually incoherent, beams. Second, we observe the symmetry breaking of the vortex-mode soliton and the formation of a dipole-mode soliton, as predicted by the theory.Theoretical results. We consider two incoherently interacting optical beams propagating in a bulk, isotropic, saturable medium. The model describes (2+1)-dimensional screening solitons in photorefractive (PR) materials in the isotropic approximation [5]. It represents a great simplification with respect to a more realistic treatment taki...

2+1)-dimensional optical spatial solitons have become a major field of research in nonlinear physics throughout the last decade due to their potential in adaptive optical communication technologies. With the help of photorefractive crystals that supply the required type of nonlinearity for soliton generation, we are able to demonstrate experimentally the formation, the dynamic properties, and especially the interaction of solitary waves, which were so far only known from general soliton theory. Among the complex interaction scenarios of scalar solitons, we reveal a distinct behavior denoted as anomalous interaction, which is unique in soliton-supporting systems. Further on, we realize highly parallel, lightinduced waveguide configurations based on photorefractive screening solitons that give rise to technical applications towards waveguide couplers and dividers as well as all-optical information processing devices where light is controlled by light itself. Finally, we demonstrate the generation, stability and propagation dynamics of multi-component or vector solitons, multipole transverse optical structures bearing a complex geometry. In analogy to the particle-light dualism of scalar solitons, various types of vector solitons can -in a broader sense -be interpreted as molecules of light.

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