Alexander Dreischuh

Observation of attraction between dark solitons

We demonstrate a dramatic change in the interaction forces between dark solitons in nonlocal nonlinear media. We present what we believe is the first experimental evidence of attraction of dark solitons. Our results indicate that attraction should be observable in other nonlocal systems, such as Bose-Einstein condensates with repulsive long-range interparticle interaction.

Two-dimensional self-trapped nonlinear photonic lattices

We predict theoretically and generate in a photorefractive crystal two-dimensional self-trapped periodic waves of different symmetries, including vortex lattices-patterns of phase dislocations with internal energy flows. We demonstrate that these nonlinear waves exist even with anisotropic nonlocal nonlinearity when the optically-induced periodic refractive index becomes highly anisotropic, and it depends on the orientation of the two-dimensional lattice relative to the crystallographic c-axis.

Light bullets formation in a bulk media

The conditions for the formation of a light bullet pulse, propagating with nearly constant spatial and temporal parameters in a bulk nonlinear medium, are obtained. A pump and probe pulse configuration using an induced phase modulation (IPM) is used. The IPM effect provides a well-controllable spatial and temporal trapping of the probe pulse. The authors derive analytical expressions for the critical parameters of the system, incorporating input power, pulse duration, pump and probe beam radii, medium parameters, etc. The system parameters analyzed are rather general and include varying pump and probe pulse durations and delay. The constant parameters of the light formation are shown to be limited by the walk-off effect. An analytical balance condition is obtained for the spatial and temporal soliton-like formation. >

Experimental reconstruction of nonlocal response of thermal nonlinear optical media.

We study experimentally and theoretically the nonlocal response of a medium with thermal nonlinearity and show that despite its inherently infinite range it can be accurately characterized by a well-defined nonlocal response function. We retrieve the shape of this function and analyze its transformation with the change of boundaries.

Generation of lattice structures of optical vortices

We demonstrate experimentally the generation of square and hexagonal lattices of optical vortices and reveal their propagation in a saturable nonlinear medium. If the topological charges of the vortices have identical signs, the lattice exhibits rotation, whereas if their signs alternate between being the same and being opposite to each other, we observe stable propagation of the structures. In the nonlinear medium the lattices induce periodic modulation of the refractive index. Diffraction of a probe beam by this nonlinearity-induced periodic structure is observed.

Spatial phase dislocations in femtosecond laser pulses

We show that spatial phase dislocations associated with optical vortices can be embedded in femtosecond laser beams by computer-generated holograms, provided that they are built in a setup compensating for the introduced spatial dispersion of the broad spectrum. We present analytical results describing two possible arrangements: a dispersionless 4f setup and a double-pass grating compressor. Experimental results on the generation of optical vortices in the output beam of a 20 fs Ti:sapphire laser and the proof-of-principle measurements with a broadband-tunable cw Ti:sapphire laser confirm our theoretical predictions.

Laser-induced non-sequential double ionization investigated at and below the threshold for electron impact ionization

We use correlated electron–ion momentum measurements to investigate laser-induced non-sequential double ionization of Ar and Ne. Light intensities are chosen in a regime at and below the threshold where, within the rescattering model, electron impact ionization of the singly charged ion core is expected to become energetically forbidden. Yet we find Ar2+ ion momentum distributions and an electron–electron momentum correlation indicative of direct impact ionization. Within the quasistatic model this may be understood by assuming that the electric field of the light wave reduces the ionization potential of the singly charged ion core at the instant of scattering. The width of the projection of the ion momentum distribution onto an axis perpendicular to the light beam polarization vector is found to scale with the square root of the peak electric field strength in the light pulse. A scaling like this is not expected from the phase space available after electron impact ionization. It may indicate that the electric field at the instant of scattering is usually different from zero and determines the transverse momentum distribution. A comparison of our experimental results with several theoretical results is given.

Polychromatic nonlinear surface modes generated by supercontinuum light

We study propagation of polychromatic light near the edge of a nonlinear waveguide array. We describe simultaneous spatial and spectral beam reshaping associated with power and wavelength-dependent tunneling between the waveguides. We present experimental verifications of the effects predicted theoretically including the first observation of supercontinuum nonlinear surface modes.

Supercontinuum generation with optical vortices

We employ an optical vortex beam for the generation of femtosecond supercontinuum in a solid state medium. We demonstrate that the continuum generation process is initiated by the filamentation of the vortex, resulting in a spatially divergent continuum. Despite the strong self-focusing and the formation of multiple hot-spots along the vortex ring, the singularity is preserved in both the near- and far-fields.

Interactions of optical vortex solitons superimposed on different background beams

The interaction between two optical vortex solitons (OVS), formed on different background beams is analyzed numerically. Analogous to the one-dimensional case, vector OVS seem obtainable [12]. The relative topological charges of the interacting (off-axis) vortices are found to rule their propagation characteristics. Attraction is found in the case of equal charges, in contrast to the opposite case, where repulsion is present.